I think he has a strong hand.

He check called a bet and a raise on the flop. That could be a draw. But if he wanted to semi bluff the flop would have been the time to do that with a draw not the turn after you miss and after your opponent is likely to have trips.

He could have the jack. But he didn't bet the flop.

A set would make some sense here. On the flop he slow played it. Now he figures after a bet and a raise on the flop someone has trips. He wants to bet out and get a call from the trips. He doesn't want a scare card like a heart or a K,Q,8,7 for a straight scare to come down and shut down someone who has trips. He figures the other two are not likely on draws because they bet out the flop. This seems like a scenario that fits. This fits j9 also I think.

full house:
55: 6 ways
99: 6 ways
j9: 3 ways
total: 15

jt: 4 ways
qj: 4 ways
kj: 4 ways
aj: 3 ways
total: 15

Looks like the full house is about as likely as trips if you can make those assumptions about playable hands preflop. And the semi bluff doesn't make much sense.

The bet is almost 3x the pot. You don't need to catch a bluff often with that kind of bet to stay in positive expected value.