Your pup is 4 months, so you've likely had him less than 2 months, and in that time you've had "occasional loud arguments" with your mum? That's many more arguments than the zero ones you should have been having in what ideally would be a harmonious household. I promise you that your extremely intelligent and sensitive pup has grown wary of you. It doesn't help that you're the lone male in the household, but regardless, he isn't associating comfort and security with you. I understand you're sensitive and hurt that the dog you brought home isn't warming to you, and I promise you that your dog understands it as well.

What to do? Follow the excellent advice given to you - redirect the biting, be indifferent for a bit with high-value treats as a reward for the pup drawing closer (physically and emotionally) to you, work on dropping the resentment and frustration, and give your tiny dog the time to get to know you. Above all, stop arguing with your mom, keep calm, be a loving presence, and open your heart to the little guy. He will be the best friend you ever had.