When Matilda was the same age she adamently refused to drink out of a cup. We just persisted with one sort for a week. We even mixed whole milk with formula to see if that would work for her. Does he drink milk out of the bottle? Maybe try to get him used to the flavour as it is different to formula before trying it out of a cup. MAybe with half formula & half milk for a while and warmed as usual. Goodluck!