My son keeps on clearing his throat and coughs. .for 1.5 months now

[Fernando_slp34]

About 1.5 months ago I had the bright idea of taking him with me to a job a was doing staining  a trim and sanding it,  i had my son about 40 feet away, but I think that was what affected him because since that day on he’s been doing this.  I even mentioned this to the docs and they say nah I think your over thinking it.  I’m frustrated with myself because my son was super healthy now he looks good but he keeps having that issue.... doctors said his lungs, ears throat and heart look/sound good.  But I can tell something is wrong he never used to do this before.  I feel like a failure I need to take him to specialist or an alternative doctor or both. :(. Some one please help me if you have a similar case.