I hope someday he will submit to having his body handled out of trust and not just for the treats.
At 4 months old I wouldn't worry about that yet. It takes time to build a relationship.
It's much easier to start working on handling desensitization when a puppy is smaller - with Dena it was an exercise in her puppy class, which she started when she was 16 weeks old, so it was a bit more challenging. With Keefer and Halo I started at home much sooner. With all three of them, I used a LOT of treats!
I agree that it's best to go slowly, with frequent short sessions. There's a digital download of Dr. Ian Dunbar's book After You Get Your Puppy that talks about handling. It's available on the DogStar Daily website: http://www.dogstardaily.com/files/do...Your_Puppy.pdf
You need to create an account in order to view it, but it's free and just takes a few minutes. There's a ton of great training articles on the site, so I'd highly recommend you sign up. This book was used as a textbook of sorts for the Sirius Puppy program developed by Dr. Dunbar, and were the puppy classes I took with Dena, Keefer, and Halo.
Here are a couple pictures of me in class with Halo:
As you can see, I've got her cradled on my lap, with one arm around her chest while I touch her ears, feet, tail, belly, and face while feeding her treats. It's important that you do not let the puppy go if s/he struggles, wait them out and mark and release as soon as they relax, even for a second.
From here, we worked on having the puppy on their side and gently rolling them over. We also did tons of work with collar grabs for a treat, so they don't learn to avoid a hand reaching for them. You can say something like "gotcha!" in a happy tone of voice as you hold the collar and feed a treat and then immediately let go. Make a game out of it so it's fun for him.
Halo was used as the demo dog by the trainer to demonstrate this:
As far as the humping, I only have experience with one male dog. Keefer isn't a huge humper, he did hump Dena when he was a puppy, and both he and Halo occasionally hump either other in play, but he's never done it to strange dogs or inanimate objects.