Re: Ideas for increasing calories...
I second the Greek gods yogurt; that stuff is amazing!
And I'd keep offering avocado, both in smoothies and plain.
The most calorie-dense foods DS can't get enough of are olives and nuts, but he's obviously much older than your LO. I wonder if you could make an olive tapenade and spread it on things (like rice rusks). Nuts you might want to wait a while on, but of course nut butters are full of good fat and protein.