Will also post on child health.
DGS has Cp which is not the issue. He has always struggled with his weight, just not a foodie unfortunately. 4.5 lbs at birth. He was going along OK and slowly gaining weight, but after a recent cold with lots of vomiting has lost weight for the first time.
He now weighs 10 kg (22 lbs) is age 2.10 and on the 50th centile for height (normal height) his BMI is now 11.3 
We have all the supplements under the sun and add butter, cream cheese etc to food. The dietician said loads of high fat calories. We have discovered mascapone (fabulously calorie laden!) but wonder if anyone else has any suggestions for REALLY high calorie food, so we can feed him up again.
PS CP really not the issue here, and we are going to get him checked for tongue tie but in the meantime PLEASE list the highest calorie foods possible to get his weight back up.