I would NOT suggest the Alpha Omega Elite, ESPECIALLY if your son is tall. My oldest had this seat, and was too tall for it before he was 3yrs old, and well before he was the 40lbs that the harness goes to. It is possible to outgrow a seat by height long before weight, and a 3yr old is NOT MATURE enough for a booster with the adult belt, you NEED to remain in a 5pt harness. The Alpha Omega is an ok seat, jack of all trades, master of none. Its a short seat, the very top harness slot can only be used in booster mode, and again most kids outgrow it in height by age 3 or so, and way too young to be in a belted booster. Plus its a veyr wide seat, meaning in booster mode it will hold hte belt too far away from the childs hips, especially a thin child, and the seatbelt then cannot do its job properly in a crash. Its not a bad seat, its just not the best seat, and certainly not the 'last seat you'll never need' like they claim.
THe good thing about the Britax seats is they harness to 65lbs, but they also have one of hte highest top harness slots on the market, meaning your tal lskinny 5yr old will still fit in the Marathon perfectly. My 6.5yr old is 51'' tall and 50lbs and is just now outgrowing the MArathon in height. We have the Marathon in the Racer cover and the Mossy Oak cover. I have very hot blooded kids and neither have ever sweat or had issues with the covers. They are super easy to remove (like a shower cap, stretchy) and wash beautifully. I can't stress enough the importance of the extended weight/height limits of the Marathon either.
Another good choice is the Evenflo Triumph Advance. It will rear face to 33lbs, and forward to 50lbs, and fits most 5yr olds. My oldest could use this seat at 5. Its a bit cheaper than the Britax Marathon, and you will get about one year less use out of it height wise. But its still a nice comfy seat, a great deal for an extra 10lbs and few inches to grow.
Just make sure you keep your child rear facing to the rear facing wieght limits of 33 or 35lbs. The AAP recommends children remain rear facing to 33lbs nad 2yrs old. My 3yr old just went forward facing a couple months ago at 33lbs.
Look for a seat that will install in your car correctly, and better yet find a CPST to install thes eat for you, and gain some education in the process. Keep your baby rear facing to the weight limits of the seat, and forward facing in a harness until AT LEAST 4yrs AND 40lbs, but better to harness longer.
Also to the CPST who doesn't like Britax because its hard to install rear facing in small cars.... we have a Chevy Aveo, a sub compact car that is the hatchback model, and I had our Britax Marathon installed rear facing in there for our 3yr old, didn't even have to move the front seat forward, its approrpriately reclined (and I was able to get 45degree angle for a baby) with no problem. If I can install it in THAT car, it should easily install in any car. I have pictures if anyone would like to see.