Waking up at night

So last year my son slept through the night had a bed time routine and was fully independent at playing by himself but in October last year my father in law decided to move in with us because of his living situation and our whole living situation flipped upside down we had to rearrange our rooms and it was a mess he would wake up at 3 am and start his day but when he did move in my son started waking up at 3 am every day going into my father in laws room and hanging out then would fall asleep at 10 am and sleep till 12 and be up and we didn’t find that out till months after it started happening anyways it continued till he moved out in February and we finally got our house back and my child will no longer play independently like I knew would happen because he always had a play mate in his grandfather and now 5 months later still wakes up every night at 3 am 2 weeks ago his schedule got so outta whack cause he would stay up from 3 am and not go back to sleep till 12pm 1 pm and so me and him had to cycle our schedule all the way back to sleeping normal so we do it and our schedule is he goes to bed at 9:30 usually out by 10 and then he has been wide awake at 6-8am which is not great but it’s okay but he is still waking up at 3 am to come to our room the tv is off and he usually goes back to sleep but how to I get him to have a full night sleep in his bed without waking up? He stopped having naps at 2 years old I believe if not earlier it’s just never been his thing but now he has a day or 2 when he will nap but on the days that he doesn’t he is trying to fall asleep by 7 and so we have to keep him up I just want him sleeping in his bed till 6 I know it’s not sleep regression because it’s been happening for almost a year now but before my father in law lived with us he would never wake up at night an was sleeping from 10 pm to 9/10am an I just want that back lol any tips we are trying the stay in bed till the light turns green with a alarm clock thing but he isn’t fully awake when he is getting up at 3 so he doesn’t even look and we are usually asleep as well so we don’t see him when he comes in I have a camera so that’s how I know it’s 3 am so it’s not like we can put him back into his room he is exhausted when going to bed idk what else to do we tried to melatonin but it isn’t that he can’t sleep lol and he isn’t heavy enough for a weighted blanket he is 27 lbs we can’t even do a 3lb one yet any suggestions? It’s not the falling asleep it’s the staying asleep lol