Well he is 8 years old right? First mistake you made was points. You said you don't want to head him in a direction and let him follow his own path. Guess what? You are directing him in the wrong direction. Your direction.
That said, you didn't say if he knows this is the points you are getting him or he understands or not. There are 3 ways to play
AoS but you are only showing him one way. Why? Because you mentioned points.
If he wants points let him then. But guess what? I will bet you at 8 years old he doesn't care or UNDERSTANDS points. Let him choose what he wants. Will explain more in a bit, want to make sure my next question is correct.
Does he really want Stormcasts? At first I thought you were going to say what army to start with him, but then said you bought him Stormcasts and I am thinking to myself "you didn't say he wanted them so you already started him with something that is not his choice" and then later said that is what he wants. So not sure if he really wants them or another army. Just wanted to make sure he wanted Stormcasts.
That said, just let him choose what you want and say you can't afford it if he picks minis that are too expensive.
Now onto points. He is 8. He doesn't really know the meaning of "savings" for money so he will not really understand about points either. I am talking from experience here. While he might know about saving a penny or two, or more about a quarter here and there, he will not know how to save a $100 or $500 or know if that is a lot of money. So he will not understand 100 or 500 or 2000 points.
When my son wanted to start Necrons in
40K, I tried to give him the codex and show him the points and told him to make a list on what he wanted, he just said "I want to play these models" and I said "but you need to make a list" and I basically lost him there. Then it hit me, he doesn't UNDERSTAND what it means to really make a list, or why he just wants to play with cool minis. He just wants to roll dice. So he doesn't care about lists or what not. That is what Daddy is suppose to do.
When he grew older, he just lost interest in the game because the rules were crap. I kept him interested by using the Lord of the Ring rules into
40K. But now he has no interest at all at 15. List making? Forget it. Reading all these rules and books for
40K? Forget it.
But this is about Age of Sigmar. I just wanted to show you I am talking through experience of a young kid who started painting my minis at 3 years old
LOL. Don't worry about points, just let your son take what he wants if you can afford it. I had to tell my son, NO a lot of times sadly because "Daddy didn't have money to buy it." and he understood. He was happy with what he had.
Modify the rules when you need to. Remember he is only 8. Let him have fun. Let him choose like you said. Don't make the mistake I made thinking he needed to learn "properly" at a young age so he wouldn't make mistakes.
Now I might be going of topic here now, but will say this. While yes it is about him having fun, let him learn how to loose properly. When my son was 3 and wanted to get into Daddy's toys and play, I let him play with me. He always won. He had such a blast. Then one day we were at a
GW Headquarters store when they had them. He herd 2 nerds talking smack to each other. He was 5 or 8 or so at the time. He started talking smack to them on how "he is so great at
40K, he never looses". The look that 2 guys gave him like he was a little kid and didn't know anything pissed me off so much that we had to leave.
"Why are we leaving Daddy?" he asked. "I am about to punch them out for looking like you were nothing". It's ok if nerds talk smack and think it's fun but then when someone sees it and wants to be like them and then copies them and then they have the nerve to look down on him instead of trying to encourage him more just pissed me off to no end. What made these nerds think they were so great with plastic toy soldiers and were better than an 8 year old? If they can talk smack why can't a little kid then? Hypocrites. :p
Well that taught me that he needs to know how to win graciously, but also needs to learn how to loose as well. So he got upset when he lost his first game. He wasn't happy at all, but I told him "I don't complain when I loose all the time." and he went "no Daddy you don't" so now he learned how to win graciously and loose like a gentleman.
So I thought maybe I need to teach him how to play "properly". Worst mistake I ever made. The rules for
40K are so horrible, that my son lost total interest in the game, like I said before. He would only play when I make the lists, an do all the "paper work".
So while you need to make sure you son needs to know how to win and loose graciously, don't really worry about points and let him play what he wants. After all he is 8, it's about cool minis and dice rolling. Mathammer comes later and it's not for all people. Young kids shouldn't have to worry about mathhammer and that is what you will be showing him that can turn him off the game since he doesn't see that part of the game right now.
So don't worry about points. Just do what shows your son interest and just go in that direction. Great thing about
AoS is that there is 3 ways to play. Unlike what happened to my son, when he just wanted to play, and I believed the internet crowd saying you need to play
40K with points and it lost my son interest.
If you son doesn't want to play with points, just play like how
AoS was originally supposed to be played. Just take what you wanted and roll dice. So to keep your son interested in playing
AoS just let him do what he likes.