Help with wee brother please?

Ok, he's not my real brother, my family went through a foster to adopt program and he's currently living with us, he's 5 years old and he's settled in really well. I said to him that we could do something fun this week/weekend (it's a school holiday here) he seemed pretty excited, and said something about going to a nearby indoor waterpark, then he stopped and said no never mind. When I asked him why, he went quite and mumbled that he's scared of people seeing his marks. The reason he was put into care was because he was beaten, and while most are unnoticeable, he still has a few marks that you can see. I ured him that he wouldn't be alone, as I have had hospital treatment in the past resulting in scars, however he's still weary. I don't want to give up on the idea as at first he seemed really excited about it. Any ideas how to convince him?