When my dd was a baby she was always small and gained weight very slow. My ped told me to give her more bfing to gain weight if under 12 months of age and give more food to gain weight if over 12 months of age. With that said, she never was interested in extra feeds. Think Denver would take in more formula? Not sure how else you can get extra calories into him... if I remember correctly- is he taking the formula for MSPI babies? Can you make it thicker (use extra, if it comes in powder form)?
For solids, I would say to go for the calories- are you giving the baby jars of food or making yourself? If you're using the jars- always read the calories when you're shopping and skip the low cal stuff like carrots. I recently bought jar food as we were going to be out and one brand of squash was 80 calories for the jar and the organic one was well over 100 calories for the jar (first brand had more water mixed in). For foods you're making offer higher calorie choices like avocado, banana, if he doesn't like those see if you can "sneak" them into other foods. If he eats meats go for the fattier stuff- ie. when giving chicken don't give chicken breast, instead give the dark meat and meat from the back and legs.
Keep in mind at this age los want to move around and seem less interested in foods, it's common for their weight gains to drop off a little.
