loss of weight from constipation

My DS#2 (Braxton) has been constipated since he weaned at 7 months and went on to formula. It has gotten so bad lately that the doctor prescribed laxatives for him. Since then, about 2 weeks ago, he has lost 1kilo. The doctor suggested we take him off the formula and put him onto cows milk to stop the constipation but it hasn't worked. He now tells us that we have to put the weight back onto Braxton but I can't get him to eat anything. I am open to any ideas since the doctor has given me none. The only suggestion he made was to give Braxton Metamucil as it is 100% fibre and will help him not be constipated but won't make his food go straight through him. Any ideas on meals that will help him put on weight and that are high in fibre aswell?

If you're giving metamucil you need to up the amount of liquid he's drinking because increased fibre without increased liquid can just make things get even more stuck.
Maybe you could try fruit smoothies - they have fruit and liquid in them (you might want to skip the banana smoothie because banana gives some babies constipation). Just put yoghurt, milk (or formula) and some fruit in a blender and whizz it up. I'm not sure if Braxton is old enough for honey but if so you could use a bit to sweeten it. You could even boost the fibre a bit more by sneaking in some wheatgerm.
Porridge (made with full cream milk not water) with a bit of gloden syrup and milk on it is yummy and high fibre and all the milk and golden syrup might make it more fattening.
I've posted in the cooking section to see if any of the foodies can help out. The topic is here

I also came up with a slightly whacky idea - maybe if you're any good at expressing you could give Braxton a bit of EBM in his formula because it can have a mild laxative effect.