Gift For Classmate`s Birthday

My preschool son has been invited to attend the birthday of a classmate. We appreciate the invitation but cannot attend. Should I send a gift?

It`s a huge class that my son will attend again next year. We are already part of a large playgroup, and we don`t plan on inviting the entire class to his birthdays. Is it okay to get gifts for only those parties we can attend, and is it okay to NOT invite the entire class to my son`s parties (there is a school directory, so we can invite friends without bringing invitations to school)?