If you're worried about the ' thing' on his side then it's a two person job, get someone to hold/restrain him & then examine him & get it sorted.. The titbit idea to distract him is not a good one as you will have ti time it correctly i.e. only give him the treat when he shows the wanted behaviour & not when he's growling as this will only reinforce this behaviour.(& his anxiety) likewise, do not stroke or talk to him when he is showing aggression.
A good way to educate him to accept to be examined is just before a walk ,when he has other things on his mind, just pretend to examine him for a few seconds & then go walkies then repeat this ritual every time before a walk, increasing the examination' by several seconds each time or until you see he starts to be a little uneasy........that way he will associate you fiddling about in his fur with something pleasurable & positive.