Big 3YO - HBB ok?

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DNephew is a chunky just-3 year old, he's about to outgrow the car seat he's currently in (Graco Turn2Me) as he's just under 19kg and 104cm (limit is 19.5kg and 105cm). Still RF at the moment out of habit but parents are happy to FF or RF. I drive him several times a week for various appointments/pickups etc.
DB/SiL don't drive for medical reasons and will pay for a new seat to use in my car. Together we're trying to come to a decision. Ive got a 5 door fairly modern car with all the isofix, top tether options etc. Nephew is very sensible and wouldnt mess with the belt on a HBB, will sit up properly etc if told to (rare for 3 i know!).
Looking at options HBB start from about £40 and harnessed ERF Swedish type seat are in the hundreds, so its a big difference (they'll need at least 2 of whatever we land on as DGPs drive him too). FF harnessed seats he's too heavy for except the odd one that goes to 25kg - does it make much difference at this point (prices are at least double most HBBs)?
Can we go straight to HBB as he is well above the weight and height minimums? Or do they need to pay the £££ extra for 2 harnessed FF/RF seats (they will pay it if needed but wondered if it was safely avoidable due to CoL).

Despite his size his bones aren't mature enough to withstand an impact using just the vehicle seat belt.

A 36kg ERF seat, although more expensive, would be safest and last the longest. The Britax Safeway M is £170 odd.

Hi OP,
I’d echo what the PP said above, at 3 he’s too skeletally immature for a HBB, plus can you guarantee he’s emotionally mature enough to sit still and not play with the seat belt?
If his parents are happy to keep him RF, he’d be much safer until the age of 5 when he could be turned FF into a HBB.
Not sure what cars you are dealing with but the Britax Safeway M is currently on offer for £170, and the slightly upgraded Max Safe Pro is £200. Both RF to 36kg, and if you purchased a spare set of tethers it would make moving the seat between cars much easier.

It would definitely be worth keeping him in a harness. The forward facing seats convert to HBB so will last longer but they are all fairly basic in terms of safety features so RF is clearly the safer option.

Definitely would not put a just 3yo in a £40 HBB if I had a choice about it.