The answer to this question depends on a few factors, including the current laws and policies related to military conscription (i.e., the draft) and the status of your son.

First, it’s worth noting that the United States has not conducted a military draft since 1973, following the end of the Vietnam War. However, current laws still require all male U.S. citizens and male immigrants between the ages of 18 and 25 to register with the Selective Service System (SSS), which maintains a list of eligible men in case a draft is ever needed in the future.
If your son is male and between the ages of 18 and 25, he is likely required by law to register with the SSS. Failure to register can result in penalties, including fines and restrictions on federal benefits.
In the event that a military draft were to be reinstated, the SSS would use a lottery system to determine which eligible men would be drafted into military service. However, there are certain exemptions and deferments that could impact your son’s likelihood of being drafted. For example, men with certain medical conditions, religious objections, or who are the sole surviving son of a family that lost a member in military service may be exempt from the draft.
It’s also worth noting that there are currently no active plans to reinstate the military draft in the United States. While there have been discussions and proposals related to expanding the pool of eligible military recruits, there is no imminent threat of a draft being reinstated.
In conclusion, while it is possible that your only son could be drafted if the U.S. government were to reinstate the military draft, current laws and policies related to military conscription make it unlikely. However, if your son is a male between the ages of 18 and 25, he is likely required by law to register with the Selective Service System.