helping boyfriend deal with childhood trauma

my boyfriend was spoken down to a lot and abused regularly as a kid. he told me once his dad smacked him across his face cause he said he wished they could stay at their cousins house longer and once he was crying because he hadn’t seen his mom in months and as punishment his dad didn’t let him see her the next time he was supposed to and plenty of other similar stories. this has caused him to be very quiet and afraid to speak. often times i feel like i have to walk on eggshells because the smallest thing i say just reminds him of something from his childhood. this morning we were fine and having a normal conversation as he was making a sandwich and i casually said “that’s so much” when i saw how much jelly he put on his sandwich, obviously i just meant it’s more than i would’ve used, and he immediately got upset and said “i don’t even want it now” and left the room because it made him think of when he was a kid and would always get yelled at over the smallest thing like putting mayo on both pieces of bread when he made a sandwich. i always am very sympathetic, but i know if it were me upset because he said i used too much of something he’d say i’m being dramatic. i never say he’s overreacting even when i think it. but sometimes i feel like the smallest thing sets off some kind of trigger. and i understand that he went through stuff as a kid that i haven’t, but i feel like he projects it all on to me because he can’t say anything about it to his family. i want to help him deal with this but i don’t know how, i feel like it’s not healthy to be this touchy. and i feel like he’s not like this with his friends, just me. and it’s hard for him to move past it cause his dad is still (i would say) mentally/emotionally abusive to him even in his 20’s. he calls him lazy and selfish all the time. he works for his dad and has him in the position where he’s doing all of the work for his dads business and his dad doesn’t handle it, because his workload is so heavy his dad makes it his job to run the business though he desperately wants to quit and get a different job (especially because he makes less than minimum wage) but he knows his dad would react negatively and blame him for any business failure, but he still always talks about how soon he’s going to kick him out knowing that he doesn’t pay him enough to live on his own. i feel like it’s very hard for him to move past his issues when his dad still treats him poorly. i’ve suggested he go to a therapist on multiple occasions but he doesn’t want to.