Yes, your son meets the definition of a qualifying child. One of the requirements for a 21 year old is that they must be a full time student. According to the IRS, one qualifies for full-time student status if they were enrolled full-time for a period of at least 5 months. Since your son was enrolled from January to May, he meets the definition. It is important to remember that for purposes of the Child Tax Credit, he does not qualify as he is over 17, but he is a qualifying relative so you may be eligible for the Credit for Other Dependents.
For more information on dependents, please see Rules for Claiming Dependents on Taxes.