>Problem is he has only EVER had dried food,<
Is this because he is not used to fresh foods,or because he gets unhealthy table scraps which cause the runs?
It doesnt make sense to me that fresh wholesome meats,veggies etc would give a dog the runs,but eating nothing but dry kibble is keeping him
100% healthy

I would start by adding some raw green tripe,it is great for adding body plus it has an abundance of enzymes which amongst other things help them to efficiently digest and utilise the food,i would start adding small amounts of grilled chicken,beef,fish,lamb and veggies working your way up to decent amounts and then maybe start adding very small amounts of pasta and rice(remember dogs are primarily meat eaters).
I would make it my priority to start getting healthy fresh foods into your dog,his overall health and vitality will benefit greatly and you will no doubt start to see healthy weight gain particularly muscle mass,which i should add goes hand in hand with adequite exercise,How much does he get daily?
What dry food is he on now? I have heard people have had success with arden grange for putting on weight.However a better food IMO with a nice protein to fat content is timberwolforganics wild n natural,which is what i'm feeding my crew on at the moment.
Ingredients:
Chicken Meal, Fresh Chicken, Chicken Fat (preserved with mixed tocopherols and rosemary extract), Low Ash Salmon Meal, Potatoes, Sardine/Mackerel/Anchovy/Tuna Oils (preserved with mixed tocopherols and rosemary extract) Eggs, Dried Chicken Liver, Dried Whole Milk, Dried Whey Extract, Kelp, Alfalfa Leaf, Casein, DL  Methionine, Potassium Chloride, Lecithin, Taurine, Lysine, Carnitine, Choline Chloride, Creatine, Probiotics: (Lactobacillus Acidophilus Fermentation Product, Bifidobacterium Thermophilum Fermentation, Bifidobacterium Longum Fermentation Product, Enterbacter Faecium Fermentaion Product, Bacillus Subtillus Fermentation Product, Blueberries, Cranberries, Mixed Tocopherols (a source of vitamin E), Zinc Proteinate, Iron Proteinate, Manganese Amino Acid Chelate, Thiamine, Niacin, Vitamin A Supplement, Calcium Pantothenate, Riboflavin Supplement, Vitamin B12 Supplement, Iodine Proteinate, Vitamin D3 Supplement, Biotin, Folic Acid, Pyridoxine (a source of vitamin B6), Cobalt Proteinate, Papain, Yucca Schidigera Extract.
Analysis:
PROTEIN: 36%
FAT: 18%http://www.zooplus.co.uk/shop/dogs/dry_dog_food/timberwolfArden Grange
prestige
Fresh Chicken (min 19%), Chicken Meal* (min 19%), Whole Grain Rice, Whole Grain Maize, Chicken Fat*, Beet Pulp, Egg Powder, Fish Meal*, Dried Brewers Yeast, Linseed, Fish Oil*, Minerals, Vitamins, Nucleotides, Prebiotic FOS, Prebiotic MOS, Cranberry Extract, Chondroitin Sulphate, Glucosamine Sulphate, MSM, Yucca Extract. *Preserved with mixed tocopherols and rosemary extract.
Protein 30%
Fat 21%http://www.ardengrange.com