From robotnut's Billy Blastoff webpage.
He was kind of a chubbier, baby-faced, Major Matt Mason.
This was an awesome toy, and probably has a lot to do with my interest in space exploration.
He had two AA batteries in his backpack (like air tanks), and they caused plastic pistons that extruded from the bottom of his feet to oscillate, causing him to walk forward (More like a shuffle). They also caused a crankshaft and cog strategically located in his anus to rotate, which, when he was seated in his various space buggies, would cause them to move. It was really pretty ingenious.
And this thing was durable. I had him til I graduated from high school, and every few years I'd take the corroded batteries out and put in some fresh ones, and the darn thing still worked fine. By that time, his helmet-visor was smashed, probably from an Alien facehugger.
Artist's conception: Billy Blastoff vs. Alien
The top photo isn't a complete set; he also had a little orange sled that could be hooked as a trailer to two of those vehicles, and a yellow TV camera the same color as the ray-gun, and a rod with a scoop (for moonrock collection), also molded from yellow plastic. He may have had a couple of light-up devices that plugged into his suit as well, but if so, I lost those pretty early on. Here's a 30-second YouTube of a TV commercial for this set.
Even as a kid, I'd long since forgotten that this guy was called Billy Blastoff. Now, it took me awhile to find him on the web, through various google searches that were variants of "What was that chubbier, baby-faced toy that was kind of like Major Matt Mason?"
Major Matt Mason toys were really before my time, but I had a friend in the neighborhood who had some. They might have been hand-me-downs from an older sibling, or maybe his parents got 'em at a garage sale.
The figure that stood out was Callisto, an alien who had a gun that fired elastic string, sort of like toys I've seen to annoy or amuse your cat.
There's also a good Major Matt Mason webpage here.