Medication has bad effects on boyfriend?

he's really a great guy. i'd like to stay with him long term, but after 2-3 months of dating im seeing that his medication DOES have effects on our relationship. his meds make him less responsive, when he forgets to take them i can tell because his response time is more in pace with mine. like if i make a joke...ON meds, he may just chuckle with disinterest or take a few moments to actually take in what i said....off meds he laughs and comes back with something equally entertaining. they make him spacey and at times moody...his behavior seems like a muted version of himself. im TRYING to be supportive because i know he needs to be on them....but what can i do? he also gets very defensive and hurt when i bring the issue up.