Can you try wrapping his arms before you start?
Or try feeding lying down on your side with bub on his side - I found that at least meant I didn't have to worry abut bub falling anywhere while I sorted out hands and mouth and boob.
Also maybe try giving him the other side first every feed - sometimes until he's finished it, and other times just for a couple of minutes until he's got the hang of mouth-opening.
Sounds silly - but if you feed more often then he won't be as hungry and may be gentler too.
If you have to, keep getting help until it's OK.