I would be sure to clean it the day you leave thoroughly and wash all his toys and the tank walls, do a 100% water change and fill it to the rim with water. Use conditioner that balances ammonia - it may not help much but it will help a little. YOu can also get an ammonia blocker and add to the water according to the instructions. It is not the best solution but it is better than ammonia building up.
When you get back, do another 100% water changes as soon as you arrive.
In fact, you may want to make it easy and prepare the water you will need for when you get back and keep it in another container so when you get back all you have to do is change the water without all the prep work.
If you have a secondary fishtank, you can prep the water and whatever else you need so you just plop the fish over from one to the other as soon as you get back, then you can rest easily without worrying about him and clean his tank in the morning.