I think that all Merlin has as he waits for Arthur are the memories, every little moment with him. And they don’t dim over time, he makes them stay and live and breathe as he experiences and has adventures in the new world. The memories grow to the point that when Arthur finally comes back he’ll stand in front of Merlin and the poor guy will think it’s a dream until he realises it’s not and he becomes so happy he launches himself at Arthur and gets his hug.
Merlin gets to take Arthur to lunch because some of his first words after thousands of years of nothing is ‘I’m starving’ and he makes Arthur taste a burger for the first time and see his face when he falls for them. Then three days later he regrets it when Arthur has to get new 'weird future’ clothes and Arthur is so huffy because he doesn’t understand sizes of clothes and has to get a large and he thinks that 'Even the future tells me I’m fat, remind me why I woke up again Merlin?’.
And Merlin tries to explain, even goes so far as saying that Arthur can have any burger he wants because he may have lost some weight from being dead for thousands of years (at least he hopes so and that a thousand years has taught him to lie well enough to fool Arthur) Except Arthur still knows because, apart from hiding the fact that he is a Sorcerer, that Merlin can’t lie worth shit to Arthur and he throws a burger at Merlin while cursing his high metabolism, which explains why he such a skinny stick.
And Arthur is terrified of the zips in the jeans after failed attempt number 5 of putting on clothes by himself in the changing rooms because he claimed that he was still a King and can manage to dress himself in outfits that don’t look as complicated to put on than his armour and tunics. Merlin can’t stop laughing at Arthur’s pained squeaks and Arthur goes 'It’s not funny Merlin!’ and tries to punch him in the arm except now Merlin’s out and proud of his magic by now and manages to dodge, making the rug tug out from underneath Arthur’s feet and the once and future king crashes to the ground yelling 'That’s cheating, what have we talked about Merlin?’ and Merlin helps him up still laughing saying 'Upsie Daisy your Lordship’. And they both start laughing until Arthur does this snort that reminds Merlin of the Donkey Ears and they have to support each other because they’re laughing so hard and it’s the longest and hardest they’ve laughed in millenia.
Then there’s snow and Christmas and they have a snowball fight where Arthur takes out any and all of his frustrations (mainly television) over the new world by pelting Merlin with snowballs and Merlin feels victimised; 'Stop throwing snowballs at me! Fine, see if I wait for you next time you prat!’ and Arthur just taunts him by saying 'What you going to do to me Mer-lin?’. He is buried in a ton of snow later and they have to spend the rest of the holidays in their rented flat shivering by the fire with colds because while Arthur caught the cold he still managed to pass it onto Merlin in revenge. Merlin doesn’t want to cure himself because they only have one blanket between them and is much happier snuggled against Arthur side while they watch Christmas movies.