the best way to find his weight would be to go to someone who has a large scale used for weighing grain (feed mills and many times a farmer who grows grain)
or a good vet clinic or vet school will sometimes have the scales too. If that is not possible, try to get someone who is experienced in estimating weights of animals to help you. Weight tapes are nearly useless as they can be off by a great deal even for larger horses.... things that can help you decide if you have overloaded your critter: does his back sag under the weight, does he have a hard time keeping his balance. Does he huff and puff after just a little work, does he show resentment at being asked to work, is he sore the day after you use him? All those are strong warning signs that you are asking too much too soon, or just plain asking too much! make haste slowly in conditioning and the horse will get in condition with fewer injuries and stay sound longer.