Re: Ideas for increasing calories...
I second the coconut oil!
We had my DD on a high fat/high calorie diet when she was around 9-10 months old. She actually used to eat coconut oil off of a spoon -- she loved it! I would offer her a few spoonfuls after she drank her milk; it really helped a lot (in keeping her weight steady).
Also, pureed prunes are calorie dense and are great for preventing constipation!