Missie's 8 years old and still hates her harness (she only wears it when we go out of the house), so I imagine it's pretty normal. Have you tried different types of harnesses to see if maybe one is more comfortable for him? I've heard others here mention vest harnesses being better.
For now I would just do sessions with harness and leash inside (depending on what your vet says, like Chachi said). I would use desensitization techniques with the harness and leash off first, so he associates their presence with something positive rather than scary. Then put the harness on for short intervals throughout the day and give lots of praise (and a few treats) once it's on. After he's able to remain calm with the harness on I'd put it on every time before he eats so he associates it with good things and can focus on something else while it's on.
Then do the same with the leash. Pull it out often and praise him for sitting next to it or touching it. Lay it by his bowl while he eats. Once he's got that then clip it on and repeat what you did with his harness.
Time and patience will work wonders.
My other little dog, a Chi mix, loves his harness so much he can't contain himself when I open the drawer it's in! He associates the harness with walks and car rides, so he'll do everything he can to HELP me put it on. So that's what you're aiming for--a positive association to the harness and leash. Good luck!