How old is the sibling? Old enough to get some info from? Even a 3yr old might be able to give you some info maybe try asking questions such as what is the boys favorite toy, does he like loud noises, does he get scared by loud noises...see what you can get to work with.

There is the chance the little guy has just shut down. He may be so overwhelmed he's distanced himself from everyone and everything to cope. "If I don't hear/see you you're not here."

You can also hint at it at pick-up. I notice little johnny doesn't startle to loud noises like most 15month olds...is this normal for him or is he trying to settle in and is tuning things out. See what they say. There isn't much you can do...you need him fully settled before you can get a good sense of what the issue is, you can't diagnose a child on how they behave in a new environment especially if it's a huge transition. Hopefully the child is just tuning you all out and in a matter of days he starts responding!!