Agree to continue with Metacam and keep him comfortable. If he started to lose weight then you might need a dose adjustment so the dose wasn't too high for his weight - that's the main thing to look out for.
Not sure where you're going away on holiday, but if it's self catering, I've often taken an older hamster with us
. If that's not possible, then leaving them in their cage in your home with your Dad feeding/looking after him is a good option too. You could have a phone call or three with your Dad so he could put it on loudspeaker and Loki can still hear your voice as well.
Out of all the things I've tried when going away, the above two have worked best. What didn't work so well for us was taking the hamster to someone else's home. They seem better left in their own environment with someone going in to do the feeding etc. Is that possible? I think familiarity is the key when they are older. Familiar surroundings, familiar voice etc.