I have had a dyslexic player/game master in my group for over 5 years. He's great and super creative. I find with a little bit of pre-game prep (getting spells and abilities he most uses written down in a way that is easy to understand for him) and help from our group's rules guru for difficult events (like critical roll with a magic weapon while smiting) everything runs pretty smooth. When he DMs and we as players write off-screen blurbs we have 'story time' where the writers narrate for him and anyone else in the call at the time. We have played Savage Worlds, DnD 3.5 and DnD 5e. By far the biggest challenge for him was 3.5 because it has SO MANY rules and books. 5e and Savage Worlds are both simple enough that once you get the gist of things you rarely have to crack open the book (And Google is your friend!) Probably the best thing about 5e and Roll20 for dyslexics is that a lot of things can be copy/pasted as spells, etc into your character sheet. So if the DM has a question if a spell can do that and your boyfriend just can't quite dig through the text, just by clicking the spell/ability name it posts the spell description on Roll20's chat for everyone to review word-for-word and make a quick ruling.