You're basically on the right track with getting his respect and control of his body, as you described in your second paragraph. I think it's just going to take some refresher courses for him. Horses are a constant maintenance project, behavior-wise. You can't fix something and never go back to it and have it stay fixed. Even manners get rusty sometimes.
I recommend reading up on Clinton Anderson's groundwork, if you haven't already. He really emphasizes having complete respect from the horse, being the unquestionable leader without being "mean" about it, and actually does it in a way very similar to what you were already doing.
Some little tips and tricks I've picked up:
Only turn towards the horse, don't pull him around you. Always a good safety tip, but doing it constantly (and on both sides!) really drills the idea of personal space into their heads.
At feeding time, bring the food to him and demand that he wait until you give him the food. I've seen horses cultivate major respect issues from being led to a stall where feed was already waiting. I almost goad mine into being snots every now and again so I can see what needs work. It's a good feeling when a horse will politely leave his grain and allow you to fill a water bucket in the middle of his "meal," lol.
With the sunscreen, try approach and retreat, rewarding even a slight attempt to stand nicely. This method also works great for dewormer, fly masks, almost anything.
If he's being nippy, be passively defensive. Keep your elbow up a little higher, ready for him to ram into if he swings his head.
I would suggest that you and your husband get together a bit and try to work through your horse's issues, especially if he is more experienced. This is something that you should at least have some experienced input on in person.
I hope that was somewhat helpful, and good luck!
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown