At 21 pounds, he is really at the bare minimum to go forward-facing; here we like to ask, "why do just the bare minimum when it comes to safety?"
Your Alpha Omega is a convertible carseat; meaning, it CAN rear-face (to 35 pounds, highest rear-facing weight limit on the market), then it can forward-face (to 40 pounds, or when shoulders go over top harness slots, which are the second-to-top headrest setting).
Do you have the base on or off? It can be removed if it helps the install.
Your 88 should not have locking belts, as far as I know. What infant seat did you have installed?