but ditch the flash and get a rugby pelham with a long shank and nice thick, flat bridle to compliment his head
Agree with the flat noseband and no flash, but def not changing his bit to a pelham just for the sake of it.
It would be worth trying on on him (try a cambridge mouth - they're generally well accepted by the horse) and use it for shows if he likes it, but if he's happy in his snaffle I'd just leave him.
And again, I don't think he needs any more weight. More muscling, yes, but certainly not more fat!