Why is my husband suddenly jealous?

okay... my son is a young teenager, he has a girlfriend, she's the same age, a couple of occasions they have gone out together to parties, her father, who is bringing her and her brothers up is a single parent, and collects my son, brings him back, he's very friendly, a nice guy. he collected my son today, my husband was looking out the window, though this chap didn't see him looking, i was seeing my son off, had a few words with the dad, and apparently when i went to walk back in the house he was 'eyeing me up' so my husband says, i thought he was having a laugh, but he was serious, it's not that i give any encouragement. i have noticed in the local shop where i bumped into him once that he was looking at me that made me very flustered, but i thought i must be imagining it, my husband is jealous and insecure, does this man fancy me like hubby reckons? it's made me feel very shy