Player advice

Hey everyone, my brother and I are new to 40k (started about 1 1/2 months ago). We started with the leviathan box (he got the space marines) and now I'm picking up thousand sons because I absolutely love their lore and theming. I already have about 900 points worth, but my brother has said that he will not play against my thousand sons because he feels magnus is too strong, unless/until he can get something like an imperial knight. I don't want to not play my army, so if anyone has advice on ways to win a game against magnus, that would be great. Or should I just pull magnus and run without him when I play my brother?

To be fair, I see where he's coming from. We did a small fight, magnus vs 500 points of space marines and he completely wiped them, but still. Any help is appreciated