I believe the Chazon Ish says you shouldn’t let a baby cry.
Maybe there’s something comforting you can give him – something familiar, a toy (a mobile), blanket, a water bottle (ppl don’t recommend any other bottles at night) etc, or stay with him til he falls asleep. (Actually this has the opposite effect for me – what works best for me – I put my baby in, give her a blanket & pacifier & leave immediately, otherwise she gets excited that I’m there & it keeps her up.)
Good Luck!