One thing that struck me is that you mentioned he isn't eating much hay and isn't as enthusiastic about his nuggets? Can you remember roughly when that started? The reduction in hay consumption and struggle with eating hard food is a strong indicator of teeth issues, possibly tooth spurs. If his mouth is feeling sore, that may be one factor explaining his aggression.
I would get a vet to have a look at his teeth. Please bear in mind that they cannot check the back teeth during a conscious examination (as they won't be able to comfortably open the jaw wide enough to see) so you may need to get a full dental done under a GA. I appreciate that seems a bit drastic but, if dental issues are brewing, it's better to nip it in the bud now while he's still able to eat for himself and in good enough health to undergo the procedure.