Is it normal to not have Asthma in the summer?

My step son comes to live with us in the summer, and visits us in the winter. We live in Minnesota and he is in Texas during the school year. During the school year he is sick often and has been diagnosed with Asthma.My question is, is it normal that he is absolutely fine when we have him in MN, but needs to go to the ER for steroid shots during the school year and is on allergy and asthma meds and occasionally uses a nebulizer....He exhibits absolutely NO problems with breathing while he is with us and is not taking any medications.