I don't see "we" can diagnose, or even guess with so little information.
But, I suggest if you son was symptom free for the first 29 years, it is unlikely he has asthma. Look for any changes in his diet and activity in the last couple of months there could be a clue there.
yes he could have developed asthma but he also could have an URI. he really needs to go to the doctor and see what is going on. because he may need steroids and antibodicts , the doctor may prescribe him some inhalers.