and asthma- can well be triggered by house hold dust- especially from rugs/ mats/ mattresses/ carpets/ molds etc.
If his problem was due to allergy/ asthma- its wise to have a clean up. When its being done he has to be kept outside.
There are preventive medications to control the frequent attacks of allergy/ asthma. This can be discussed with his doctor.
If the previous attacks were upper respiratory infections (URI)- it has to be kept in mind that upto 6 episodes of URI per year are normal in school goers.
If he is growing well for his age and is active, there will be nothing much to worry. He will outgrow these eventually.
Hope my answer helps you.
Wish your grand son good health.