All forward facing harnesses have to meet the same standards, so a combination seat (5 point harness then booster later) is just as safe as a convertible seat that's used FF. At 2.5 your son is on the younger side to ride forward facing, but if he's already forward facing and outgrowing his convertible seat there's absolutely no reason to buy another convertible.
The frontier is a great seat, especially for tall children. It has the tallest top harness slots of any seat on the market (20") and should get any child to a safe booster age before they outgrow the harness. It then becomes the tallest high back booster on the market; by the time it's outgrown your son will fit safely in an adult seatbelt without a booster.
The nautilus is a good seat. The top harness slots are 18", which translates to about 2 years sooner that he'll have to switch to booster mode. My kids have insanely long torsoes (I've run into very few with torsoes as long as theirs) and it would have gotten them to at least 4.5 before boostering (which is on the young side, but fine for a mature child who will sit correctly). It becomes a decent high back booster and also becomes a backless booster, so it will be the last seat your son will need.
I don't know a ton about the proride, but I know it's a fairly popular seat with carseat techs. If I recall correctly (and I may not on this one) it has top harness slots about halfway between the nautilus and the frontier.
You also might want to look into the evenflo securekids series. They have the same top harness slots as the nautilus and become good boosters as well. They're the narrowest combination seats on the market, which may or may not be a concern (don't know if you ever need to get three people across a back seat, which is when that matters).