My son also likes to breathe with his mouth and that in turns lead to frequent throat infections.
We used nose drops for him to help clear the nose so that he can breathe with his nose. We used either Otrivin or Iliadin. Both can be bought over the counter at Watsons or Guardian.
Then when my son's nose is clear, when he is fast asleep, we will go into his room to close his mouth. After doing this for a few nights, he kinda subconciously got the idea that he should breathe with his nose