Difficult to say without seeing him but around 10.5 stone maybe slightly more. It depends on the rider, how well or not he is put together and the type of work being asked of him
If you go by the show cob rules a light weight should be capable of taking up to 13 stone, and a heavy weight cob over 13 stone.
These weight are based on bone circumference, measured just under the knee, rather than how chunky the cobs are, but often these two things do go hand in hand.
Bearing in mind these types of cobs are up to height 15hh, would expect a 14.1hh to comfortably cope with 12 stone?
As above poster says it also depends on the work required. I have a 14.3hh cob that happily carries me out hunting all day and I weigh 10 1/2 stones. He is strong as an ox and imagine he would be equally happy carrying somone of 12 stone when fully fit and muscled.
I'd expect 12stone but remember weight of tack! My lad is 14hh and has 8.5 bone , he's carried my OH a few walk laps around the school and wasn't bothered but I wouldn't do it everyday.
As others have said depends on a lot of factors. My boy is 14.1 and has 10 inches of bone, he is built like a tank, but he is hyper mobile and we are working on building his strength to keep him stable and balanced at the moment.. He has just turned 5 and we are only now moving on to schooling moves in walk and trot. I waver twixt 8.5 and 9 stone and wouldn't want him to carry any more than that at the moment. When he is fully developed and fit I think he could probably manage a lot more.
I suspect some of you don't realise how much a normal taller rider weighs. I'm just over 5'8 and slim : size 10-12 and lower healthy end of the BMI scale. I weight over 11st with riding gear on.
I very much doubt if any of you thought I'd be too heavy if I rode your 14.1 cob. Tall maybe, heavy no.
With no conformation faults (long back, weak loins etc) I'd expect him to carry 12st easily. He'd prob carry 14st without complaint too but I wouldn't want that weight on him regularly for long periods. Just because they can doesn't mean they should!