I find that if DH and I eat dinner with her at the table, she will eat more than if it's just me or if it's just her at the table. Like tonight we had a baked chicken dinner and she ate most of it before tipping it onto the table and rubbing it in....
And I was thinking if you wanted to try with the smoothies, maybe get a cup with a straw, like a magmag cup, so even if it is a bit chunky he can suck it up.