If your son is interested in lizards, would you consider a snake?
Snakes are much easier to keep than lizards, in terms of set ups and feeding. 'Beginner' species such as corn snakes are placid and easy to feed and handle, and an eight year old child would be able to do everything themselves with adult supervision. Feeding is once every 5 days or so for babies, once weekly or fortnightly for adults, and cleaning is as and when needed, usually a few days after feeding.
Set up for a baby consists of a heat mat, thermostat (very important!) thermometer and a large flat 'faunarium' or converted (air holes) plastic shoebox or similar. You can use kitchen roll as substrate and empty toilet roll tubes as 'hides' to keep costs lower and cleaning easier.
You could easily leave a snake for a week, or even two whilst you went away, although it is maybe best to get someone to check that temperatures were remaining stable and water wasn't empty if it was longer than a week. Even my non snake loving friends are happy to check in if they don't have to open or touch anything.
You do need to do some proper research first, but snakes are one of the lowest maintenance pets you can get IMO.