Well...it can be Extremely hard to saqy if he will need it, because one big factor is going ot be how mature he is, which obviously you can't know until he is older.
Is he currently nuerotypical?
If he is curently nt, without physical or other issues, then chances are he will likely be mature enough for a booster by age 6 years old.
What size are his parents? Do you believe he will continue to grow big and tall, or might that slow down a bit?
For example, my dd, who had the same size torso at that age, DID have a torso and overall growth slowdown. She went from literally off the charts big, to riding the 95th percentile..to riding the 75th percentile...and finally settled on the 5oth percentile. We sort of expected this since, although she was a GIANT baby, dh and i are NOT big people.
She had a 14 inch torso at 18 months, but yet now, at just turned 5 years old, her torso is 16. So, only 2 inches of torso height in 3.5 uyears. She is even with the second to top slots on a nauti, and therefore has a full 2 torso inches left in it...probably get her close to 6.5-7 harnessed, which is more than enough.
Even if his growth DOES continue to be insane, chances are a nauti will still have high enough slots to get him close to age 6. Which is plenty old enough, and big enough for a booster.
So...will you *need* a regent. Probably not. And, even IF you got him a regent, made TODAY, it expires when he is 7 anyway, which means at BEST, you'd be buying him maybe a year in it over a nauti. If you got one that was any older than brand new, you probably wouldnt get ANY use out of it over other seats on the market.
Also....4-5 years down teh road is a LONG time..a REALLY long time in the carseat world...there will be dozens of new options between now and then. I don't think getting one will be worth it at ALL, sorry.