I'm in the same position of having done this ages ago and trying it now. The saltwater technology has changed radically: 1. instead of a sealed system in which you try to prevent evaporation, you try to make it evaporate up to a gallon a day for a 55gal and replace with fresh water via a float switch. 2. instead of a filter, you use a sandbed and live rock with a protein skimmer to get the last of the crud out. No more filter changes, just emptying the skimmer basket and replacing. 3. You use a sump about half the volume of your upper tank if you can find the room, which lets you make your changes/site your skimmer in safety: water cycles down to the sump and up. There may additionally be a refugium, a small area with live plants, sand bed, and light, for more processing of waste products. 4. because of the evaporation rate (and the growth rate of the critters) you need to test more. An easy way for your son to learn chemistry. 5. you do have to do at least monthly 20% water changes to maintain and prevent buildups; I run 10% weekly, which works out better. 6. Lighting is more complex, and in the case of corals, pricey. What else? Those are the major changes. NOW: back in the day, we ran a closed system salt tank stretch 30 with a penguin filter and a high-output fluorescent. We had to clean algae like fiends, but the fish were happy. You have to test every few days and still do those water changes, but for fish and a very few mushrooms or button polyps, those conditions will work. Getting a fish like a lawnmower blenny will at least help with the algae.
I hope that helps. On the one hand the pricey system is lower maintenance, but more bookwork and learning; and on the other, the other is more intuitive and less expensive. It all depends on what you want out of it...now if 'for my son' adds up to 'we' and you want to get into it, certainly you might find it fun, and get in over your head just like the rest of us.
