Esther has good advice. I would add that it would be worth your while to take him out on lead each time till he understands that he's to go outside. If he is loose and on his own when outdoors he may not be going potty.. just playing. There are a lot of distractions... try to keep his attention on potting .. a treat for him may be that you let him play outside once he's gone.
When you catch him going inside... just calmly say no ~ outside! Pick him up (interrupt if need be) and take him out. Wait till he finishes going and praise him for going Outside!
Keep him on lead when not crated so you can rush him out the door quickly. Someone once told me not to clean up the mess in their presence but I don't remember why. At this point I'd probably be willing to try anything.
One last story ... at one of my many appointments when Chessie was a pup my vet asked me how potty training was going. I said GREAT! She hasn't had an accident yet! He was very surprised and pleased to hear it was going so well till I added.... Nope no accidents... she does it on purpose!