K.H.
Hi
My 3 yr old daughter does the exact same thing , she always comes into our room and climbs into bed around 2 in the morning and falls pretty much straight back to sleep , she was in a crib converted into a bed and so I thought that maybe the bed was the issue as it isn't very wide and she rolls around alot and has fallen out a few times. So we put her into a regular sized twin bed and so far so good (she has been in the new bed 2 weeks) , we have had a few nights where she has woken and come into our bed but I have carried her straight back to her bed and she has stayed put , I think she now just needs to get out of the habit. Obviously I don't know if your son is in a toddler bed or not but if he is then this could be something you change to see if the issue or not.
Good luck
K.