Can you read a book to him or have a toy there for him to play with? Anything that may distract him.

My DS is pretty good getting his hair cut now but at 1st he would move and cry, we just talked to him to get his mind off what the hairdresser (my SIL) was doing and read stories etc. He loved the idea of being in the big chair too instead of the kids chair he was usually in. Could your DH go along too and get a haircut to show DS that its ok?