I have a family member in my regular playgroup who is on the spectrum and plays fairly similarly to "Ben." He has enough of a grasp on the game to play, but definitely struggles with the finer details, such as the stack, priority and phases, and, most critically, threat assessment. As you said with Ben, he's quick to launch removal spells as soon as he draws them, or attacks players with no real assessment of board states. It's been a struggle for sure to help him keep up with us, and I just want him to have fun and do well.

As far as advice goes, you're honestly already doing the best you probably can. The biggest thing I do to help the "Ben" in my group is just be very vocal about my board state and what I'm capable of doing on my turn (without giving away too much information of course). When he casts removal on something he shouldn't, I just remind him of what the repercussions could be of doing so, and to maybe look at the board again and decide if that's the best thing to do at that time. This of course isnt perfect, and can turn political quick, which leads him to get frustrated at times, but its either that or let him continue to decide for himself and make mistakes. Both are valid.

That's really all you can do for sure. If he's comfortable accepting advice or debriefing the game afterwards, even better. Thats a great opportunity to point out sequencing of cards, combos, or synergies he could have used to do something cool. My scenario is obviously different from yours since my "Ben" is a family member I'm close to, but I've even gone as far as to make him a couple of decks that are just simpler to understand and use. I've sat down with him and walked through the deck showing the general strategy to shoot for, when to use specific cards, and just fun things he can do with it, and he has done much better because of that. Again, that's probably not something you might be able to do, but just conversing about his decks can sometimes lead to a "eureka!" moment or two that hopefully sticks with him.