It may not be laziness, it may have happened by accident once or twice and if you are sure it is nothing medical, than you need to go back a stage, it takes about 3 days (give or take) to make/break a habit in children and if he has relaxed enough for it to happen a few times then the muscle memory/brain relationship needs to change, just like whe you pt him.
Firstly I would look at how he acts when he has done it, you say he acts not bothered, are there any other behaviour concerns, I'm not meaning to be alarmist, at all it's just this could be something in a bigger picture. But presuming it's not and this is isolated behaviour there are two ways I would deal with this and they really are dependendent on your DS and the relationship you have with him...and how he responds to rewards/you etc...
And the ideas merge so you could pick of bits from both, but basically if he is happy and comfy to be nakad, I would have him bottomless (if that makes sense..) in the house, as this removes the feeling of something tight like a nappy/pants and does nt allow his mind and his body to relax enough to think/feel it's safe to do a poo...obviously if he reacts to being nakad, this won't work for him, so you could try just trackies/loose trousers - again it's about removing the association of the pants/nappy muscle memory and re;triggering his brain and body to think oh I have to go to the loo do to this...the second stage of being nakad is putting the loose trousers on, then the third stage is introducing pants, you want to do/have 3 days of success at each stage before moving to the next stage and if at one of the stage he has an accident go back to stage one...
Obviously amongst all this, which I'm sure you are already doing; you need to give the accidents no association, not positive or negative, make no comment at all (which we all know is very hard)
Anyway suggestion number 2, is a good old chat/heart to heart and tell him how it's making you sad and you thought he was a big boy bla bla and you don't want to out him in nappies, but you are not sure what else to do and then together come up with a plan....(you obviously jolly him along to this point) then choose a wow toy that he can have if he does a week of accident free, at the end of each day out a star on a chart and very over the top work towards the goal....
Hope that helps......