Is this the same very underweight horse from your other thread? If so and if those pictures are recent I would suggest not riding him at all until he approaches a much healthier weight. That will be best for him and you. Depending on what you are feeding him to put weight on it might be making him a bit hot as well, which wouldn't help the situation.
The mare I recently bought was a bit underweight- about 75 pounds or so (with protruding hip bones) on a petite horse- and I ended up just putting weight on her the first month and doing groundwork with her. The groundwork allowed us to build a good foundation first and when we did start riding I certainly appreciated the wait. She was feeling better with a healthier weight and we were both feeling good after working on the ground. She is also a very forward horse that liked to have her head up like a giraffe, so I focused a lot on slowing her gaits, getting her to respond to "easy," and praising her when she dropped her head while we were in the round pen. And of course the respect I gained from her first is priceless both on the ground and under saddle. Good luck!