[JoeyP]

Ooh this is no good
You need to stop this as soon as possible
because it will very quickly balloon into a full grown aggression problem
Most likely he is scared of the harness and is acting out because of that

I had a Lhasa who was very afraid of a hair brush
I let him smell it I touched him with it While I was holding him
and it took a while but he finally did get used to it

By whatever means you can you need to stop that biting and get to the point where you can do anything with your dog without him biting you