I bought a chihuahua pup two months ago he is four months old now and he is still a aggressive biter. He won't let me comb,brush, give him a bath and clip his nails. I had to tie a cloth around his mouth to groom him but this take to much time. He's to frisky and won't pay me any attention when I'm trying to train him.
You may want to look into a professional dog trainer. You do not want this to go on for long or the problem will escalate. A professional in the field can offer good advice. vetcare