My 2 1/2yr old son started to pee while sleeping when I started to work. At age 1 to 2 I have no problem with that because I trained him to pee before going to bed and he's now using pajama during bedtime but recently he always pees during mid morning. At first I thought that it's just because of the weather since it's rainy season but it happens again and again. What else can I do to stop him?
At his age, it is totally normal even if he has been dry at night for a while. It isn't even a diagnosible medical condition until he is 6.
The technical term (if he were older) is secondary eneuresis. This is often caused by stresses in the child's life, like a change in schedule, or a new member of the family.
The first rule is to not get upset at him. He can't control it, and it is likely as upsetting to him as it is to you.
At his age, there really isn't much you can do. If you are really worried about it, talk to his doctor, but don't be surprised if the doctor doesn't seem too concerned. There is a possibility it is being caused by a urinary tract infection, and your doctor can rule that out, but the vast majority of cases aren't physical issues, and they do eventually resolve themselves. Until then, use pull-ups and don't let it stress you. The more stress this causes you, the harder this will be on him.