We had one or two guys come on here complaining that they developed D after starting on protein powders--especially cheaper brands. I would suggest that you stay away from those, as they may make you worse. Also, Vitamin C can cause D in some people, so be careful with supplements not to get more than 100%.
Unfortunately fat is helpful for gaining weight, but often causes IBS problems in a lot of people. Also, sugar helps add on the pounds, but can not only be a trigger in some people, but it also usually comes with side effects, like hyperactivity, raising blood sugar, and creation of pure fat (not muscle).
Here's a thought for you, though: sumo wrestlers. They bulk up eating nothing but traditional Japanese meals, which include a lot of rice, soy, steamed veggies and fish. These things are all pretty safe for people with IBS to eat (although you may need to cook your veggies until quite soft to make them easier on your guts to digest). They just eat a LOT of that. Also, muscle weighs more than fat, so weight-bearing exercise will help you bulk up your muscles and gain weight without gaining fat. The funny thing is, despite their fat (no, not all of that is muscle), sumo wrestlers have been found to be quite healthy. In fact, they usually have less internal fat (fat around their hearts and livers--considered the most dangerous) than some people who are of normal weight according to the BMI! That must mean that their diets are really good for their bodies and that the exercise and training (including mental, such as meditation) they do are also good for their bodies.
Since you are worse in the morning (I know how that goes!), try eating lunch, supper and then have breakfast before bedtime. Plus your snacks. Also, have you tried calcium? Inape on here (Linda) did a bunch of reserch and found that Caltrate 600 w/ vitamin D has the most costipating side effect of all the calcium supplements (you can get it in generic at most drug stores), and that's quite helpful for people with D. It helped me out a lot when I first started taking it; it got me over a particularly bad spell. Also, try probiotics. If you've not had any in the past, start out with a yogurt (Activa or Stonefield) and work your way up to the supplements. They can cause a lot of gas and cramping, so you have to ease into them. But they work wonders for some people and give moderate relief to a number of others. I won't take antibiotics without them; they keep me from getting terrible.