Hi there,
You are right, there is no way to adjust the junior projumps. How tall is your son and how much does he weigh? There are some 'awkward' heights and weights to be in bocking. My sister is almost tall enough for adult bocks not quite there and too light for them too. So when she uses junior bocks there are in much the same position as you have described. You can see a vid of her on them here.
Having the bocks closer to your knee does increase stability. However, some tall people have it much lower. If you anything under 5'5 you have little choice
But anyway. It's nothing to worry about. I have used the junior bocks often and I can't see why it would be a problem. I think the lightness of them means the stability is less of a problem. If he is comfortable that is all I would worry about.