M.K.
my friend's daughter who is 6 was potty trained by 2 during waking hours but is a very deep sleeper. They get an alarm and get up with her to pee every night and that keeps her from having accidents.
im curious why your son was waking up every night when your BF was living there but now he is sleeping thru the night. Did your BF stay up late/work late and that woke your son? Whatever was causing it, it seems that going to the bathroom at night solves the problem.
i think it will be important to talk to your son about this too. not so much focus on the peeing and more on how he feels about your BF leaving. he may be scared that you will leave or worried that he did something. make sure he knows you wont decide to go away and its not his fault.