Thats it. It's an overuse injury mixed in with the fact his body is growing and has grown quite a lot over the last 12-18 months. I know people think we are cursed, but honestly, young players at every club pick up niggles. Now the FA Youth Cup is done they can see that they can give him a break for 3-4 months to recover. In honesty, it will do him and Onynago good to have that time off.
My partners lad is their age and las year was exhausted. Over lockdown when he didn't have to keep getting up for school I reckon he grew 6 inches in a year. He just needed that time away from doing constant physical work and exercise to grow. It's the same with a lot of these lads, they just need time to really grow into their bodies. Stress fractures, and in honesty fractures full stop are not a major problem for sportsmen, if they are given time to heel as the bone will grow back stronger.
I know people cite Andy Murray, but Murray was nearly 30, and played through the pain for years and refused to take time out. At 17, if you take 3-4 months out the bodies capabilities of self repairing are still very strong and will recover more strongly.
I wouldn't be surprised if the enforced break Onyango and Price have will see them return bigger and stronger.