Hypoplasia is a term used to describe where a tisuue is either underdeveloped or incomplete. Where refering to teeth it is the enamel that is affected. It can be caused by tertracycline taken in pregnancy orearly infancy but this is well known so generally avoided, also lack of vit D during tooth development can cause it, but sometimes the cause is never identified. As to wether it is isolated to this one tooth or all the dentition will only become apparent with time I'm afraid, but this is depenant on what the cause is, ie if it is genetic then it is more likely to affect all teeth, whereas if it is due to an external factor it will only be those teeth that were developing at that time iyswim. The problem is is that as the enamel isn't formed properly, or may even be absent in places the teeth are more vunerable to decay. Good oral hygiene and dietary control is extremely important.
Treatment depends on the severity of the problem
For front teeth if only mild it can just be sensitivity that is a problem so dentists treat with a desensitizing solution. If worse then veneers may be required when he is older
For back teeth again may just be sensitive, or can be protected with fissure sealants.
More severe cases may require the deficient enamel to be removed anf a white composite filling material used to replace it. Often into adult hood these teeth are crowned to prolong their lifespan. HTH