Ok 5'9 135 lbs. I wouldn't advise anything right now. You're only going to be wasting your money. If you can't grow naturally, what makes you think that you'll grow and be able to hold onto new weight with sarms or anything? You're young bro, your test levels should be super high and your body should easily grow naturally. Instead of jumping ahead of yourself, what you need to do is get the basics down. How long have you been training? What does your diet look like? My friends 13 year old son is 5'9" 140 lbs. I'm not trying to shut you down bud. I just think you seriously need to focus 110% on your nutrition rather than looking for some shortcut or "secret" to putting on mass. Food is what you need.
Take a look at this thread here:
https://www.isarms.com/forums/steroids-sarms-information/diet-nutrition-basics-101-a-3314.html
Please read that and re-read it. Then apply it to yourself. Download Myfitnesspal on your phone or any nutrition tracking app and get it done. You can easily put on 50+ lbs naturally.