Weight gain and all muscle / lean mass suppliment have a notorious history due to a lot being bulked up with rubbish. If you research into them you will see the cheap ones are infact rubbish. They all come with this scientifically designed etc. Even if they are good, they may not be good for crohns.
All I can say is my mate has a j pouch, and has always found it hard to put on weight. Hes now taking Sci-mx omni hardcore and is very pleased with the results. Ive also tried their omni pro and found it really good without any bad side effects.
One thing I will say, theres no point in bothering taking any of this stuff if you arent exercising correctly. I follow proper exercise routines now and have noticed a real difference (even though they are shorter than my ad-hoc ones). Look on that website I posted above. Try and get a personal training session at the gym (30mins will be fine). I did 3 x 30mins and he showed me what exercises to do to get the results I want, worth every penny