You need to examine why this makes you uncomfortable. Is there other behavior that makes a gift a threat? Or does it make you feel uncomfortable because of a negative internal response? are you concerned about their relationship being too close because you don’t want them to be a couple? Are you jealous or insecure because you could not afford the gift? Before you react be confident that you are reacting to a valid concern and not punishing your son for a very generous gift that’s just a small overstep.