I just switched last month to a full-back booster seat for my DS4. He meets the requirements for one, but it certainly hasn't been easy. My babysitter took him one day somewhere and somehow the back has come off, just this week. It doesn't easily fit back on. This weekend I'm going to get my husband to fix it back. Just without the back, he is all over the place, leaning against the door, taking his seatbelt off before we get to where we're going. I thought it was age 4, 40 inches. And that's what he is. I don't know, I don't feel as comfortable, though. I want to switch back. I made a mistake, though. This week all I've said is, "If you don't stop, we're going to use the baby seat again." So now it's as if it would be a punishment if I go back. Don't know what to do.I've made a mistake, a huge mistake and then I made it worse!!
What kind of booster do you have? I know with mine (Graco TurboBooster) the back can come off pretty easily if you remove the seat from the car. It's not difficult to put back on, but it's a little tricky if you've never done it before. Do your seatbelts lock if you pull them all the way out and then let them ratchet back in? If so, try that to keep him in place. And if that doesn't work, move him back to the harnessed seat and tell him that boosters are for big kids, and since he can't sit like a big kid in his seat, he's not ready for one yet.