It is possible that he re-enlisted into the Horse Artillery after being discharged from Drivers, but from what you say I suspect that it is more likely that he, as a professional tradesman, was employed as a civilian blacksmith by the Horse Artillery.
The troops of Royal Artillery Drivers were normally split between several field batteries of the Royal Artillery. Consequently, he could have been almost anywhere on the Waterloo battlefield. The Horse Artillery had their own integral drivers so there was no need to second men between the two separate and distinct corps.
According to Duncan, History of the Royal Regiment of Artillery, Vol II, p.158, 525 Royal Artillery Drivers were part of the artillery with the Copenhagen expedition, so ihe could have been at the siege.
Duncan is available on-line. You might also like to look for the 'List of Officers of the Royal Regiment of Artillery' commonly called 'Kane's List', also available on-line in seveal editions, which includes the list of officers in the Artillery Drivers (but little else).