Our daughter was on a nebulizer for years.. When we introduced it to her we let her turn it on and listen to the sound.. We let her hold the tube and feel the air. We then let her hold the mask..
We let her watch us set it up.. We then told her to pick out the books that she wanted us to read to her while she had her "treatments".. We also asked her if she wanted to hold the nebulizer Tube or did she want us to hold it.. We asked this every time..
We explained to her this was going to help her get rid of her "cough".. See if letting him have a little control over the treatments will help.
On a side note.. We purchased a Hunter air filter for her room. We would change out the filter every 4 weeks.. During high mold season, we found we had to change it every 3 weeks..When she was not in her room, we kept her door closed. We also pulled all of the carpets out of the house. This made a huge difference. Not everyone can pull out all of the carpet, our goal was to make her room an air "sanctuary". We tried to keep it dusted, tried to not have too many plush animals, her bedding was easy to wash, no drapes she had only mini blinds.. . It seemed like sleeping was the most difficult with the asthma..
I am sending you strength.. Our daughter finally grew out of this in about 3rd grade..