I have been through something very similar with boytwin. He does not have asthma, despite showing lots of signs when he was younger, as his chest problems were caused by something else (diagnosed at 13 months), but until this point he also had repeated bronchiolitis.

As for treatment, does he have an inhaler? When boytwin was having problems this did help him a lot (despite it not being asthma). Also does he sound mucousy? You could ask for a referral to a paediatric physiotherapist who can show you some tips for helping him have a more productive cough and clear his chest (postural drainage, a kind of tapping massage)