Thanks for the replies. I really hope this isn't the start of silent reflux. DS is so happy though and rarely cries unless really hungry, so no sign of it yet. Last night DS was passing a lot of wind in the night, so I do wonder if it is that bothering him. I thought about trying gripe water Aishi, thanks will give it a go.
Sara I do try to burp him and I can hardly ever get any up. Interestingly though this morning I did manage to get him to do a huge burp and he settled back down and drained the 4oz bottle with no fussing or grunting. So it does look like wind could be the issue. Will see how the gripe water works and if not try a faster teat I think.