I am not sure about a harness for his age. You might want to wait until you get him home so you can measure him. When they are super young, I know ferret harnesses have needed to be used at first. But when you are ready to invest in a good harness (I'd wait until he's grown a good amount because you will probably still have to get another one in a year..they grow a lot in the first year to year and a 1/2), go for the Buddy Belt or the Chokefree Harness. Maximo sells the Buddy Belts and Lil Sis sells the Chokefree Harnesses. You can PM either one of them with questions.
Welcome to YT!! Such an exciting time!!