Agree with the others. The formula must be made the specification on the instructions to meet a child's nutritional needs. He needs to have either breastmilk or formula as his main source of food and nutrition until he's around 12 months old as any solid foods given before then are just for fun, not as a main source of nutrition.
If you are having troubles with him taking formula, can you go back to breastfeeding or expressing (I'm assuming you were breastfeeding before and so the formula issues have only just occurred)? If not, have you tried different formulas? I mean, to me they all taste the same, but I know of babies who have refused one but drink another.
I would suggest you see your GP or Child Health Nurse about this issues, but in the mean time, please stop mixing formula in his food as it can be very dangerous. If you get desperate, have you tried a syringe or spoon feeding him formula? Might be worth a shot until you figure something out.