Some good advice, along with misinformation here.
Eat more food.
99% of supplements are worthless, literally their packaging costs more to manufacture than the "active" ingredients.
Get on a solid training program. (starting strength, Mad Cow 5x5, etc)
Lean mass takes time to build, gaining 40 lbs in 4 months is pretty much guaranteed to be 75-90% bodyfat.
Creatine is not bad for your kidneys, especially when taken sensibly (5g of monohydrate per day).
Most doctors know VERY little about diet/nutrition and even less about supplements. What is taught in medical school is knowledge that is 50+ years old on these topics
I wanted to elaborate on the creatine fear mongoring. I'm sure
@feelingroovy has good intentions and I do not doubt her husband had high creatine levels when getting bloodwork done. Creatine, when taken sensibly (5g of cheap monohydrate per day) is completely safe. I have a health science background, and worked in the industry for 5 years. One of my employees, who was also a powerlifter/bodybuilder had one kidney since birth, despite not being allowed to do many things due to his condition, taking creatine was not one of them. He was monitored very closely with blood/urine samples taken every 3 months, and he never had issues, extensively researched the topic with his own doctor and others.
There is a danger in "loading" phases that are prescribed by many creatine supplements, which tell you to take 4x the dose for a period of X days. This is a marketing gimmick to make you buy more of the supplement.
@feelingroovy's husband probably had high creatinine/urea levels from an excess amount of protein in his diet, which BTW new research shows that you do NOT need nearly as much as previously thought for optimal recovery/growth. Old school of thought was at least 1g protein per lb of bodyweight per day, I would not go over 1g of protein per lb of lean body mass.
-former competitive drug free bodybuilder/powerlifter/lifelong student of nutrition/supplementation.
Edited to add: when I worked for the worlds largest supplement retailers, we received more supplements for free than we knew what to do with, I still didn't take them (besides creatine, and occasionally protein powder mixed into my oatmeal/shake if getting it from food was too inconvenient due to work schedule etc).
"By eating large amounts of protein foods e.g. meat, fish, chicken, eggs, cheese, milk and yoghurt before commencing dialysis, you will affect the buildup of urea and creatinine in your blood. An appropriate daily intake of protein should be advised by your dietician."