On the other hand, it may be easier to just keep him home (and safe) but it may not over the longterm be the best thing for him. He will likely have lifelong allergies, and you ultimately want him to be in charge/take care of it (obviously not at age 4.5).
My son started wearing a ($3000) hearing aid at age 6. Now four years later, he is in charge of it. At summer camps, he would go to a day camp at the YMCA where the kids would do a craft and then take a bus to the pool.
Then I'd be in a quandry- should I drive in the middle of the day to the camp or the pool and pick up the hearing aid (which isn't water proof)? I finally decided that, no, I wouldn't do that. Years later, knock wood, we still haven't lost a hearing aid.