has he ever been put in with a sow/gilt and they turned on him ? if yes he could have been put off is not unusual in young boars.
supervised matings with young boars are a good idea rather than running the boar with the sow/gilt.
are his legs good a boar with bad pasturns can be reluctant to work boars need good strong legs.
could he have injured his penis in previous attempts they can suffer fractures.
try him with a gilt under supervision make sure he hits the target they sometimes need guiding on the first attempt.
we have recently used a willing in mind young boar on bigger gilts the first attempt he hit the wrongun if you get my drift so we guided him out and back in then went around the beck of him so his rear rested on my legs so he could not fall out/off, it looked strange when dad walked around the corner and there i am in a threesome with 2 pigs

it was worth it though he is going to be dad on 2nd jan (his first bday). his next attempt was unaided and again going to be dad 19th jan