Hi L.,
You don't say if you have another nightly "adult" bedmate and what his/her take is......
I have a couple opinions/suggestions - you can take them for what they're worth.....
1. It's only a problem that your son is in your bed if YOU (or your nightly adult bedmate) think it's a problem. If you're able to have intimate 'happy time' in your bed before your son wakes up and everyone loves to cuddle for the 2nd 1/2 of the night then you aren't being lazy - you're being welcoming and loving.
2. Are you teaching your son bad sleep habits.... maybe, who knows? probably he can't self-soothe when he wakes up in the middle of the night. Maybe he's lonely. maybe he'd rather be with mommy. who knows???? if he can fall asleep on his own when he lays down at other times I'd say he CAN self-soothe and he's just lonely, had a bad dream, misses you.... whatever.
3. What should you do? well, the only way to get him into the 'habit' of NOT coming into your bed is to be UNWAVERING for 21 days (the length of time it takes to build a habit). So, when he comes into your room you simply walk him back to his bed, kiss him goodnight and go back to bed - letting him cry. This, I could not do, and alas co-sleepers for a bit we became. The cry-it-out method is effective and it's not harmful, but both of us worked full-time and are cranky w/o sleep. Plus - extra cuddles are kinda nice :-)
If you can tolerate having him in your bed for a couple years - then have at it. When he's 5 or 6 he can go back to his bed with some tough love - when he's old enough to logic it through in his head.
One of the things I did do with my daughter (b'cuz at some point she started sleeping like a flippy-floppy fish out of water and I had some bruising on my ribs lol - so the extra cuddles had to go!!!!!) was to get a doll and have her 'fall asleep' while trying to put the doll to sleep. So she would pat the dolls back, and lay really still and close her eyes and if she opened her eyes then her doll would wake up etc etc etc. That seemed to work - I guess it depends on how nurturing your son is and if that would work for him.
Good luck.
B.