M.L.
If it started at the sitters, you need to fix the problem there or you're not really fixing the problem. you can only do so much at home and if he's having triggers there, every day you will have to deal with asthma issues all evening and night. if they are not willing, then find another sitter!! breathing issues are nothing to mess around with. start seeing a pediatric allergist right away. they can REALLY help with figuing out triggers and asthma management techniques. our son has it too and our dr has been wonderful...he tells us tips that our ped never told us.
the plug ins or the vicks never helped us but still keep the humidifer going. Cool mist rather than warm mist is better from what i hear. even if you dont think its working, it's probably helping a little bit with keeping moisture in the air, especially with fall & winter coming. try to keep him elevated a bit when hes sleeping.
When you start to see an allergist, he/she will probably ask you what he eats, what he interacts with throughout the day, etc. so start keeping a little log and if you can, ask the sitter what he plays with, where he sleeps, what he eats, etc. could be a food trigger, dust/dander, etc. but doing this will help them try to figure it out.
Last but not least, if he's wheezing, don't be afraid to use the nebulizer! it really does help..