My lad took along time to develop he is a 17hh tb,he didnt properly fill out and mature till he was 8,i took it easy and let him develop,he is as strong as the rest of em,only difference is he cant turn on a sixpence
my 3 coming 4 year old 17 hh now and still bum high can canter ound the school, and lunge! maybe its your menage surface? big horses do take a long time to grow into themselves and get past the gangly unstrong in themselves stage. good luck