You have to assume that the player will find some way around the disadvantage of not being able to move. They still (presumably) have to invest resources in to overcoming that obstacle, but it isn't necessarily a huge deal. For example, they might prioritize buying an item that grants flight more than buying another +1 enchantment on their weapon, but they're still going to buy both anyway. So at best they should get some minor bonus that a character with normal locomotion wouldn't get, but not anything huge. Maybe a +1-3 Natural Armor bonus?
You might consider limiting the body slots they have available. Nothing that uses the "feet" slot, for example. This is kind of a big deal, and is a much more difficult thing to bypass (you pay a 100% premium to have a slotless version of an item, and only if the DM allows it anyway), so it deserves some kind of bonus to make up for it. Maybe a racial bonus feat, useful special ability, or an extra +2 to a stat?
I would suggest giving him the ability to crawl 5' as a full round action, to minimize the chances of him not being able to act at all. It's no fun being put into a position where all your character can do is twiddle his thumbs and wait to be rescued.
Honestly, I think the Construct type that's implied by "robotic jack-in-the-box" will be more difficult to balance than the lack of locomotion. That type has a lot of built-in immunities, and usually warrants some kind of level adjustment all by itself. Maybe go the Warforged route and make it a Living Construct?
