Hi there!
Though this sounds like a super cool question, I think I would advise you take a step back and try to see if there are any other projects he would be interested in that don't have such a large scale such as "finding life on mars", which would be incredibly difficult to do at a grade school level, material-availability wise.
I suggest, however, that you change to a simpler question that could follow the lines of "Can life on mars exist?" Or "Can plants and bugs survive on mars"?
Because bacteria is technically "alive" there really wouldn't be a good way to test that at this level, but the plants and bugs, you could absolutely do!!
Your kiddo is on the right track when he suggested the fish tank and plants! Awesome job! However, to make the experiment more appropriate to the project, you could pick a planet (because just saying "planets" is unfortunately too broad, and try to simulate the environmental conditions of that planet. For example, if you pick Mars, you might want to research about what the temperature there is like, what the soil is composed of, how bright it is, what the pressure in the air is, etc. Then, you'll want to try and create that at home!
Here's a link that talks about making a "Mars simulation" at home, and you could add plants and bugs to the jar to see if they survive.
https://www.thenakedscientists.com/arti ... -mars-home
https://www.nasa.gov/mission_pages/mars ... index.html
These links should provide some help for you as you try to simulate life on your mini, at-home planet! If you don't want to pick mars, just do the same for any other planet!
Let me know if you have any other questions and I'd be more than happy to help out
Stay nerdy!
lmp1341