All sweet feed means is added molasses-- I can take any senior feed add molasses and its now 'sweet feed' lol. Only thing senior feed does is break down easier with saliva-- is he an older horse or have teeth problems?
Don't go by what the bag of feed says ever.. not with no animal! The bag says what the company wants to bring them in revenue, lol. Be careful switching his feed- you can start slow by mixing the two feeds but keep the same amount you been feeding (weight or scoop) just wean him off the old feed slowly by mixing the two.
If you want to add something to help him gain a little more weight- beetpulp is good for that and things like ricebran- alfalfa will put weight on him but you said he's already hot and that will make it worst- nothings more healthy than just giving more hay and cutting out grain all together -youd need a mineral block to make up for lack of vits in the hay and grass alone though.
Lol just keep it simple and work him more than once a week and hell build his muscle back up- everywhere he is 'lacking' looks like muscle waste to me- 30 minutes lunging a day can change that-- or you can do it the lazy way and feed raw coconut oil, lol.