If you put it in the choc milk you need to be exact about the ratio and quantities so that if he only drinks 50% of the milk you know how many mls of antibiotics he's had and you can top him up with the syringe - having half doses of ABs doesn't just mean that it is less effective but it also gives the bugs a chance to develop resistance and become AB resistant super-bugs.
If it's panadol etc it doesn't matter if he has a half dose.

The further back you put it in their mouths the harder it is to spit it out.