lmao! He was ON THE PAVEMENT IN HIS VAN?! Sorry, he is so ******** (not a legal phrase I know). Never mind witnesses - if his story is that he was on the pavement but your son's bike was in the way he is stuffed. I frankly would threaten legal proceedings. Take pictures of the accident site and pics of the damage to the bike too.
(PS - Snags - witnesses are damn useful, but actually can sometimes totally shag your case - last week I was in court against a solicitor claimant with a independent witness, my client was a 17 year old chav and his mate. On paper we were stuffed, but I ran it anyway. The I/W wqas such an opinionated twit the Judge took the view he could not be relied upon. Won it without having to call any evidence!!!!!)