ohhhhh i totally understand!!! Big boy is not a beautiful round shape, but he always looks underweight. i feed and feed and feed. one thing that i find helpful is to add breeder cubes to their feed.
i am pretty sure that every country would have some type of this, but it is a specially formulated mineral rich extruded grain feed that is extreamly healthy and really helps the muscles bulk up. because it helps the foal grow and the mothers muscles, i figured that it would be also good for horses that had problems gaining weight, and it really is.
also, some horses not being able to graze constantly develope stomach ulcers. apparently when a vet scoped a number of random horses he found that over 80% had stomach ulcers.
a horse has to have grass going throught that stomach almost constantly. when it is not possible i like to use chamomile and slipery elm powder in their feed, you would be amazed at the difference and the weight gain on this!!!
i have done random checks by putting horses on this mix even if i was positive that they did not have stomach ulcers, and i found that they all picked up amazingly. so it is a regular thing to put them opn now when they drop condition or they are new. no harm in trying huh