Won’t leave house.

My six year old Son won’t leave the house. We had planned to visit family who he spends a lot of time with and loves to see. We planned just one day out or half day with one or two choices of places  we know he would enjoy but he doesn’t want to go out. He went to my parents house on Saturday a little reluctantly but he was fine once he got there ,he went to his swimming lesson very happily but since then he won’t go out. Even letting him take a favourite toy won’t make any difference. He has just started at a new school in September which he loves and I think he’s missing going. Any ideas?