There is no low cost for non-custodial parents. He can reach out to father's groups but chances are he's not going to get an attorney for cheap.
If he does divorce, he needs to make it clear to the judge he wants visitation and that he wants to be in the childs life. He will need to ask for specific days, holidays, etc and he MUST take the child every day that the court acknowledges as his days. This way, if the mother does attempt to prevent the father from seeing the child, he can ask for an emergency court hearing and hold her in contempt of court for not following the court order and preventing him from getting the child when he its his days. Basically she could get thrown into jail for not allowing the father to spend time with the child.
She CAN NOT keep the child away from him. The only way the courts will rule in her favor and prevent him from seeing the child is if..
He's a convicted felon.
He's abusive and there are police reports/arrests to approve it.
Or he's a drug user.
And last but not least. Child support and custody are 2 different courts although they are both dealt with in family court (I know confusing) So if and when he is ordered to pay support, that doesn't mean he'll get visitation. He will need to see a family court judge regarding seeing and spending time with the child although this will be established during the divorse. If he misses payments, he WILL lose visitation so he needs to stay up to date with his support payments.
Maybe you should move out and get your own place then that wont keep happening or get an older man. Mother in laws rarely like the woman their son is marrying or married to.