Many of our fellow sim racers dismiss FH5 because it falls much more on the simcade side than sim side. And, while I am a FANBOY when it comes to Forza (I enjoy it thoroughly on a motion sim rig), there are some caveats that must be stated at the beginning...
CAVEAT: While I would concur that the FFB has been improved a fair bit from FH4. especially for those of us who use a wheel and pedals, there are still a number of remaining bugs/issues. Per aka.ms/communityupdate2 posted 1/11/22, they are continuing to prioritize at least some of those bugs. On FH4, they worked nearly three years on bug squashing so they do have a decent record in that regard. Nonetheless, any new or prospective FH5 player should understand, due to the sheer quantity of bugs, this game can still feel like a beta at times. FH5 also uses the previous game engine which was developed during one of the failed Windows Gaming Initiatives where I believe Microsoft began pushing a tweaked/uniform driver model rather than the standard driver model for controllers, wheels, etc. As a result, I understand some wheels (like Simucube) still don't work satisfactorily with FH5.
NOW THAT THE CAVEAT IS OUT OF THE WAY...
In FH5, free roam driving can be fun in the same way that LA Canyons can be fun in AC. But, the real kicker in FH5, is taking a car that doesn't really handle well and modifying the tune on it until it drives the way I want it to drive. This means learning to modify the springs, antiroll bars, rebound, compression, tire pressure, gearing, aero, brake bias, etc. It can also mean swapping the driveline from RWD to AWD or swapping a motor, turbo, supercharger, etc., and then testing the result on your favorite circuit, street course, dirt course or rally course. For those who are new(er) to tuning cars, Hokihoshi (Youtube content provider) did a wonderful rundown of how to tune that applies specifically to FH5, but also to many of the other sims we might race. It is a video that I watched several times simply because I was awed at how much real world info he managed to sneak into it and how well he explained it. (I also found the majority of the info in that video to be consistent with my own experience). Not all of it is 100% correct, but it is correct enough to be valuable asset for most arcade AND sim racers.
Long story short: Enjoy the driving first and foremost. But, if you can use FH5 and some of the tuning guides to help build or increase your own tuning ability or your son's tuning ability, then you cannot help but get a LOT out of the title. And, that knowledge will benefit you well beyond the confines of FH5.
P.S. I am always happy to help folks get FH5 figured out or to start folks on their tuning journey. We have a small discord server for that purpose and you can message me for access.