Defeated elite four, but somehow missed Blue’s mega stones. Can they still be gotten?

My son has defeated the elite four, but for some reason doesn’t have Blue’s mega stones. Can he still get them?

According to https://www.serebii.net/letsgopikachueevee/gyms.shtml, one should receive Blue's mega stones before entering the 8th gym. However, we cannot find the stones anywhere in his inventory. He also doesn't recall receiving them before entering the 8th, but it's been quite a while since he first entered it.

We've tried revisiting the 7th and 8th gyms, as well as professor Oak's lab, but no luck. Blue is not there. The only place we can now find Blue is as the boss at the Viridian City gym, where we can battle him, but he doesn't give us the mega stones.

Also, what could he possible have done that would result in not getting Blue’s mega stones? Can you refuse them when they are offered to you? Could he have sold them? If he did sell them, would it be possIble to buy them back from the shop to which he sold them?

Could he have left or stored them somewhere else in the game? Is that even possible?

If he did still have them, where in his inventory would they be?