They like feeling secure. Often just holding them and gently patting them helps send them down for a nap. At 2½ months old he still needs lots of sleep. Babies that young can actually sleep for about 16 hours of sleep. Also if you ever observe a baby they aren’t always sound asleep. It is more like lots of light naps with some occasional heavy sleeping.
Sometimes it can also be the room temperature. If he is too hot or too cold or even just having a sweaty diaper can make them fussy.
One ways to tell if it is comfort. Give the baby to different people. If it stops crying with some and not others, than it is about comfort and security. If you feel insecure around the baby when he fusses, then he will pick that up and be afraid because you feel afraid.
Also if the baby is breast fed it is really hard to tell if he got enough milk. Some will suck a little while and stop and a half hour later be hungry again.