Wa alaykum ussalaam,
tl;dr: I don’t recommend spending $20 on Starbound. Will your kid like it? Probably. But he might not last very long with it.
Long version:
I have put over 184 hrs into Terraria, but only 25 hrs into Starbound.
Terraria has one world with A LOT of depth (pun intended). It also has natural difficulty scaling which is tuned to character progression. In starbound, rather than depth, you get breadth. LOTS of (seemingly) unique worlds, monsters, elements, etc. But, when you peel away the paint, everything is more or less the same.
Starbound does graphics, visual effects, and the whole space theme really well. It is, however, by no means “Terraria in space”. Crafting is made more complex and the game doesn’t prepare you well to face the world you are dropped into. Most monsters react to your presence by charging full speed at your character, giving you little time to react. This all leads to deaths that seem unfair.
When you do look for answers on questions like “what am I supposed to do,” “how do I get stronger,” “what is the point of this,” you are met either with walls of text, or nothing at all.
I stopped playing because I felt frustrated with the experience—maybe because Terraria spoiled me with all the different ways you can make progress: Collect stars at night, dive into the earth and find hearts, mine different ore to make better gear, delve the dungeon or take to the skies for treasure.
Exploring the world was interesting and rewarding. Notsomuch in Starbound.
Based on all that, I think your son might enjoy it initially. Play with some of the neat mechanics, see cool looking monster creations, build the few things that you can in the beginning. But, as he gets punished for doing all those things without being rewarded enough for investing the time to do them, he will likely get frustrated and lose interest.
If you do still want to buy Starbound, I recommend getting it on sale.
Full disclosure:
I jumped onto the starbound train early on and backed their kickstarter campaign. I even bought a copy for my brother so that we can play together. I got as far as the UFO and (basically) Planet 2. After it went 1.0, I gave it another shot. The narrative was they added was nice, but they didn’t do enough to improve core gameplay mechanics and add elements to help make the game feel rewarding to play.