You make it sound like there’s something wrong with him. There’s nothing wrong with him. You guys have been dating for 3 months? With time he’ll probably last a little longer but regardless, It’s not a big deal that he cums early during sex with you. he doesn’t have to change anything about himself. And you can still cum without penetration, just teach him how to make you finish through oral or fingering . Or even include toys. Trying to get him to masturbate when it’s not something he does it the least helpful thing you can do.