I can mainly help you with my own experience, as my DS didn't take ADHD medication for very long.
How has he got on so far? I found that it felt very strange for the first day or so, but after a few days, my body seemed to get used to it and it just feels like me but more in control.
It's important to keep eating but he might not feel like eating very much, small regular meals can help.
If he gets a headache, you can give him calpol/paracetamol for it. This should stop within a few days but totally harmless to treat in the meantime.
I found it useful to take notes/keep a diary to take back to the doctor because I wasn't sure if I'd know what I was looking for day to day. You might also want to look up something like an ADHD rating scale to fill in generally for the last 6 months and then compare to 2 weeks on a specific dose of medication.
Or another thing is if you're looking for the medication to help with specific behaviours try keeping a diary specifically about those things and see what patterns you see.
If you are noticing side effects, note when they occur - if they are happening shortly after the medicine takes effect then report to your doctor as side effects. Whereas if they are happening when the medication is expected to wear off (instant release lasts about 3-4 hours, whereas extended release lasts about 6-8 hours) this could be what they call stimulant rebound, which is a very intense resurgence of symptoms and can be unpleasant. Personally when I get this, I experience extreme tiredness (like moving through treacle) and I am also irritable and snappy. This is something you'd want to speak to his doctor about if it persists past the first few days.
I can share some resources if you're interested in the brain science side/how the medication actually works? Some people like this and some aren't interested - it doesn't really matter but for me personally I found it quite useful.