Plan out all the encounters meticulously, make them combat puzzles for your player basically.
Find a system that'll allow you to slowly trickle rewards his way in lieu of the upgrade system.
And make the bosses reliant on one or two gimmicks that while he has to figure them out, he can still overcome.
Thats all i've got off the top of my head. Oh and definantly go full ham on the worldbuilding. Maybe have some "segments" where he has an NPC companion that he can interact with, fight alongside and keep alive for future interactions.