french how often are you using the blue inhaler - every day at 3-4 puffs? I would say this is too much, and his asthma does not sound well controlled if this is the case. Blue inhaler should not be regular used and if you are needing to use it often he needs a review, as quickly as you can get one.
In the meantime the daily inhaler will start building up in his system, and should start to offer him some relief and protection. Dust the house and particularly his room every day, buy a purifier for his bedroom to ensure the air is as clean as possible. Ventilate the room.
Isolation for you, should be complete isolation, as we have done. Its the only way to keep them safe. He would be an absolute priority in the worst case, so do not overly stress, he is a child.
Getting peak flow and oximeters are a good idea too.
I have asked my dd to stop running, and to do gentle exercise only, we are also ensuring her diet is super healthy, multi vitamins and lots of vegetables to ensure her immunity is good.
I think shielding asthmatics as much as possible is essential, not necessarily with food and meds delivery organised by gov, but just doing it yourselves.
Also read that asthmatics per se, are not esp at risk apparently heart and diabetes etc proved to be a bigger risk at least in China, that is not to say we are complacent, just that we have to be more careful than others.