My daughter (4) has had many (including one today!). They always use the gas for her although they usually ask our preference. We've only ever gone for injection where there's a cannula in place already.
For her, we stay until she's under, and then they cannulate before taking her for scan. Then they fetch us when she is awake and I go into recovery.
She has premeds (sedatives) which have recently helped her go off much more pleasantly as she doesn't get upset.
She knows what happens and that she's going to get a special photo of her head. She had some practical worries this time, such as 'what if I need a wee when I'm asleep' (we always put a nappy on just in case) and was worried that i didn't stay with her when she was asleep.
Sometimes they find waking up stressful, they can sometimes be sick or get upset. Just be there for him ans support him in whatever way he needs.
It's really important for me that I'm honest with my daughter - she's going to be having scans for 10-15 years at least, so will eventually need to be awake for them.
A lot of the time, the first anaesthetic is fine, it's the subsequent ones (which hopefully won't be needed) that are harder as they knew what's coming and fight the anaesthetic more.