My Lab used to stay 100% inside,..

Brief：
He comes only for water and for a little food. He was a most cheerful and play ful dog. Recently he got pain on one of his leg and the Vet gave a medi.. 

He comes only for water and for a little food. He was a most cheerful and play ful dog. Recently he got pain on one of his leg and the Vet gave a medicine which made him constipate. We gave him milk of magnisia. Now he takes a little watter and food, prefer to stay all the time out in the wood. But he used to stay 100% inside and used to be out only for nature's call or with us.

There's obviously something physically wrong with this dog. You said he only "takes a little food". Is he still on the medication for his leg? Is he constipated? Is he still in pain. He needs to be taken back to the vet to find out what's wrong. Dogs don't change their behaviour this drastically for no reason! there mite be female on heat somewhere he is trying to find I would suggest that you need to take him back to the vet. He may be very sick. Sometimes when dogs are sick, they prefer to be alone. Please talk to your vet soon. Larger breed dogs like labs really need more time outside to run and play. I think it is cruel to keep them inside most of the time. People think the dogs are calm. They are really bored to death when penned up inside a house all day with nothing to do. We have a blue heeler named Gracie and she loves it outside. She is a very, very happy dog and will come in when she wants but she much prefers the outside. He reached puberty! He might want more freedom.He could also see things like squirrels that are outside.Do you have any other pets?If you do they might be driving him out of the house.