Yup, a handful of Black Sunnie seeds should be about right, he'll love them I bet ;) You can choose either feed, you only need to add one. I like the Mitavite Xtra Cool as it is full of vitamins as well as a good protein feed. See how it works for you and your boy and if it works for him.
One of the reasons he may put weight on and then lose it is that TB's can be very sensitive to changes in season as there can be a big difference in the nutrients supplied in grass and hay. In spring to early summer the grass is full of nutrients, then at the end of summer when the grass begins to die, there is much less nutrition in it and this continues right through winter (it is approaching winter over there now right?) Even if it LOOKS like there is plent of grass/hay, there isn't much goodness in it this time of year so you just adjust you feed accordingly. Also, as it gets colder a good rug will help keep him warm which keeps condition on him too.