You don't say what passport your son holds but if he is coming on a tourist visa or visa exempt you may have to show a return ticket. Some airlines require this before they will allow you to board & it is rarely asked for at immigration. Try googleing onward ticket & book anything out of Thailand. Don't forget that immigration may ask him for proof of accomadation, so an invitation from you & you can also be asked to show proof of funds, I think
*****
BT or equivalent.
With regard to if he wants to stay longer as a dependent, I think he would need to have been legally adopted by his step father to be able to piggy back on his visa as you have to show proof of relationship. At 15 I assume if he stays he will go to school so may be able to apply for an education visa. You don't say what visa you hold so would it be possible to be your dependent?
I think it would make sense to talk with your local immigration office to see what they suggest or call the Thai embassy in the country he is from.