Would it be ok to purchase a gently used dehumidifier. rainy season on and my son and i are asthmatic. is there a natural way to dehumidify?

Yes it's OK: There is no problem with a used dehumidifier. There are not a lot of good ways to "naturally dehumidify". For most asthmatic people, though, it's not the actual humidity that is the problem, but increase in things like dust mites in higher humidity that are the triggers for the asthma. Work on reducing exposures to these things, such as frequent washing of bedding, dusting and vacuuming.