As an asthma patient myself, I would definitely take him to the ER. Even at 56 years old, I can go from 0 to 10 just over night, meaning I can go to bed feeling fine and somehow by the next morning I not only need my nebulizer, but also Prednisone and antibiotics.
If you ever have a life threatening situation with him, call an ambulance. They can work with him on the way to the hospital, you don't have to wait to fill out all the paper work before he can be seen and it just might save his life. You won't have to answer any questions when you get to the hospital or go through different chains of commands before he can be seen. It's a lot different than when you take the patient there by car.
Good luck to your precious little one!