I agree, his back legs look really posty (straight, rather than a nice bend at the hocks). His pasterns, though not upright, seem pretty weak/long to me... seems pretty typical in ottbs.
Over all though he is super cute! He looks in great condition for only being 90 days off too! :) Hill work will also help with a weak hind end.
Every great dream begins with a dreamer. Always remember, you have within you the strength, the patience, and the passion to reach for the stars to change the world. ~Harriet Tubman