My daughter LOVES music. I play music for her a lot. If it's too loud for her at home, I can just turn it down. She does have a lot of sensory problems. Sometimes when she is out somewhere, or at school it's just too loud for her -even if it's not loud! She will smile, but have her hands over her ears. Maybe you could try noise reduction headphones ..that way he could still hear the music, but when he feels it's too loud he could put them on. My daughter wears these sometimes, and it has helped.

Hi DesiB, I agree with MyBabygirl, sounds like it is a lot for him to take in. This is ok, just work with him. Suggestion: try singing at home with music too. Make a social story for him about singing and what he is feeling/emotions. The hardest thing is to watch him have a hard time, crying, but he needs to learn how to adapt, it's the real world, it has no filters. My apologies for coming across as harsh, but just take it slow.
Fyi, my son experienced similiar responses, but he learned, it takes time. Remember every child is different, just be patient.

Is he sensory avoiding? Does the noise of all the kids maybe overstimulate his senses? Often over stimulation will lead to a meltdown. Even if he is sensory seeking that may just be too much input to handle.
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It seems like his emotions are manifesting in that way, kind of like when someone is overly emotional like anger and joy where you just cry for no real reason. Why would that exclude him from doing something he enjoys?
Maybe if he had something that he could partially focus on like one of those lil hand held box things that has buttons, switches and the like? I think it's called a fidget box? He could fidget with that while he sings, maybe it'll help.