What could ahve caused this?

So last night I was out and my dad lit a fire in the fireplace and let my 4 year old son jelp him with it. We don't have a proper freplace protection and my son was throwing in papers to the fire. This morning he woek up wheezing and was breathing quick and then would go onto coughing fits and start crying. I took him to the doc who then gave him a nebulizer treatment and then told me my son might have early signs of asth$a since my son isn't sick. So now I have to give him nebulizer treatments and prescribed singulair. What causes asthma? And could my sons helping with the fire yesterday caused this? What should I do if the night treatment doesn't help him sleep? This scares me so much bc it isn't something I have experienced before and nobody in the fam has asthma