Treats and toys are a good distraction to help him get used to associating the harness with something positive and praising the behaviour you want but do be careful to get the timing right otherwise you end up „rewarding“ the biting.
My dog, now 6, still hates having his harness put on and I still haven‘t figured out the reason. (He runs back to his bed or just lies down and rolls over, it drives me mad!!
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You could try going back to the beginning and practise putting it on and off when he‘s reasonably relaxed. Put it on the floor with a treat next to it and let him sniff it and investigate it. You can hold a treat in one hand and very slowly start to slip it on, praising him all the time. Once he starts biting, take it away and completely ignore him and try again half an hour later....If you can practise it before you‘re actually intending to go out, you can take your time and get him used to it while its quiet - once the lead is in sight and he‘s clocked on that its walkie time, he‘s probably more wound up anyway.
This bitey stage is quite normal but the best strategy is always to stop what you’re doing, turn your back and completely ignore him so that he learns that biting isn‘t a good strategy to get your attention.
Best of luck!