Thanks all.

Part of the feeding clinic is with a dietician who gave me a list of things to add to DS diet to increase calories, but try as I might I'm fighting against DS's aversions to soft, mushy food. He wont eat soft fruits, such as bananas. The dietician suggested peanut butter, but DS wont have a bar of it, as with avocado. The last couple of days we've been trying him with fruit smoothies and so far, so good; he's even tolerating banana in it, unlike the banana custard I made for him. The issue is when I try to increase the amount he's eating, he then starts to equalise himself and will eat less of the next meal.

I may get some polyjoule, or similar, just give it a go and see how it pans out. DS has low muscle tone so developmentally he's a little behind with gross motor skills, but otherwise is a very happy little man.