My Level 3 discus & hammer coach says that as well as a basic fitness and stability routine he like to do plenty of cleans, supervised (or safety) squats, butterflies (bent arm), standing disc rotations and, specific for shot- incline press.
Unfortunately (in the northern hemisphere at the moment) you aren't going to gain much useful muscle built now for the remainder of the season. Currently our group do lots of throwing and a little "near throwing weight" medicine balls work. An over 20 male will throw a 2kg discus and a 7.26kg shot- therefore we'd use no more than 2.5kg medball for discus drills and up to 8kg for the hammer/shot drills. As the season closes (around September in the UK) then the volume of full throws will slowly decrease and the medballs will be called into play more and more. This is useful as some athletes also have an indoor winter season and thus have to double periodise their training.
Looking at Azaar's profile I think he's 14, so wouldn't advocate heavy loading of the spine yet, so in winter I would strongly advise that he and his coach devise, as our learned Aussie fitness professional recommends, that he work to improve and "make solid" his centre-line strength through leg and core strengthening, with the addition of the specific exercises listed above. As Bossman says- fuel the growth and the athlete will feel the difference- from there any throws coach worth his salt will be able to transform the winter solidity and specifically strengthened areas into the explosive power (developed through february and March into growing improvement April to September (2009 if he starts working hard this August)
As an under 17 next year that will mean 1.5kg discus and probably 5kg shot. I'd personally be training him with 1.75kg standing discus (20-30 throws on non-competition weekend, with full normal weight throws in the weeks) and for the shot using a slightly underweight 4kg medball but training exposively. Technique and explosion will be the factors getting the metres right now.