Not sure if he (3 yr boy) is too young for a booster seat. I've got a niece who's 5yr and bought a second hand booster seat at bootsale, to use when she visits me (but have seen them in charity shops too..ask at the shop). Only paid 50p for it, so it'll be something you can use and put back into the charity shop when they leave or keep for their next visit. Check with someone if 3 yr olds are ok for booster seat or if they have to have a fixed car seat? The booster seat will be small enough to fit in your boot comfortably.