Based on this:
http://www.hattrick-youthclub.org/si...peedcalculator
(Enter 16 y 8d, now 4.0, wanted 7.0, skill defending, primary training defending, secondary "the other defending", what is it called in English? The one were gk+def+mf gets trained)
He can reach solid before turning 17. If he already has some subskill, it will be even faster.
That is, if his potential allows for solid defending.
If you don't have any other really good prospect, you should focus on him, Tubby. But maybe you want to find out his defending potential first (set defending or the other defending as secondary training)