If he’s three he should be old enough to understand what’s going on , you or his father (or both of you) should sit down and talk to him about the situation. Explain that his father loves him very much and wants to stay very badly , but he has to go away for a little while for school so he can better his future. No one thinks they can discuss matters like this with children but they understand a lot more than we give them credit for.
My sister who I live with right now. She lets us stay with her rent free , so I provide the food for the household , pay my own car insurance , and pay for mine and Maya’s expenses (clothes , shoes , toiletries , things we want/need). I do a lot for her too though , I watch her daughter almost every single day of the week so she can work , I cook , I clean, I do the grocery shopping , I basically take care of the household and she works and provides financially for us.