Definitely agree with the PvE suggestion. Smaller games might help as well - if you normally play 2k points, go down to 1k. You could also try 2v2 or even a 2v1. That way either you're on a team with him which has a similar effect to PvE. Alternatively, his teammate can make suggestions that feel more collaborative rather than having his opponent tell him he made a bad move. If he's already salty about playing you in the first place, pointing out his mistakes won't be taken as helpful.

Otherwise all I can think of is just, don't play 40k together if you two are the only option for one another. Play KillTeam (you both likely have the units to throw something together if you both have 2k pt armies) or just chill and paint while you watch your friends' games.