Nope, a 3yr old is not mentally or physically mature enough for a booster seat. Plus the Alpha Omega makes a terrible booster seat and should never be used in that manner as it has consistently failed every single crash test its been in as a booster seat. Don't use it as a booster.
At 3yr old and only 30lbs, he NEEDS to remain in a harnessed car seat. A forward facing car seat is outgrown when one of three things happens; the shoulders go over the top usable harness slot (the top notch on the ALpha OMega can only be used in booster mode, not harness mode), the tips of the ears are level with the seat back or the child goes over the weight limit.
You obviously ahve a newer Alpha Omega that wll harness to 50lbs. Thats great, but the height they didn't change, so its still a short seat that most kids outgrow in height by age 3. And again the seat shouldnt' be used in booster mode because its a terrible fit on most kids.
You need to purchase a new seat. I don't understand why you would need 6 seats.... unless he consistently rides in 6 cars every day, buying a new car seat for the one or two cars he rides in every day is sufficient and you can always switch it to another car if he's riding in it. Its not necessary to have a permanent car seat in every vehicle he may possibly ride in at some point.
The Graco Nautilus will harness to 65lbs and then becomes a decent booster to 100lbs. Its $150 and you can sometimes find it online for cheaper and do site to store shipping for free.
the Evenflo Maestro and Evenflo Generations 65 are a bit cheaper but will harness longer and taller kids fit them longer too. But the Nautilus is the best choice for your son.
He's not that big and he's not that tall mama. Don't make the switch, he's not mentally or physically ready. Please google Belle's Gift, Kyle David Miller for more information on Seatbelt syndrome and why little kids do NOT belong in booster seats.