I used an agency for my initial visa in Pattaya, I already had a bank account. I did not need to use the agency for my renewal which was done with the help of a friend in Chumphon.
If you check with the Thai Embassy in Canberra that are advising that no more than 2 visa exempt entries are allowed in one 12 moth period. Not sure if this will be enforced at all border entries.
Has your son always lived in Thailand? If your son arrived in Thailand using his Thai passport then these details will be recorded and he will only be able to depart using a current Thai passport
They just need to know where you are staying. If you stay overnight in a hotel they should lodge a TM30, when you return to your own home or rental accommodation then you or your landlord should lodge again. Doesn’t always happen.