apdusp:
...he is a smart boy and has curiosity enough to get interested into robots, or electronics or software, or all the three...
i would like to hear suggestions on what product should he start with and how should i proceed...
First off, I would talk to him and make sure this is something he really wants to do.
I first got interested in "robots" when I was about 6 or 7 years old; this was around 1979 - my Saturday staples were Battlestar Galactica (the original), and Buck Rogers, along with a large helping of other science fiction and such. I tried my hand at simple circuits at the same time; playing with light bulbs, wires, motors, and batteries - much of it scavenged from old toys or junk found in neighbor's trash cans.
As I grew older, my parents continued to "encourage" me - several different robot toys (Big Trak, Tomy Armatron, etc - all of which I still have and they still work!), various toy building products (Lego, Tinkertoy, etc) - and then, when I was about 10 or so, I got my first computer (a TRS-80 Color Computer with 16K of RAM, and a tape drive - plugged into my TV). I also got one of those Radio Shack "150-in-1" electronics kits. Some of my friends had computers, too; and one of them was building tube radios!
By the time I was 14 - as a freshman in high school - I had been programming in both BASIC and assembler, was on BBS's constantly - in short, I was a major geek (and still am).
I guess what I am saying is make sure your grandson really wants to do this. I'm not saying everyone will follow a similar pattern as I did (I do have to say I've met kids who are younger than I was then doing incredible things with Arduino's and Raspberry PI's, though!) - certainly one can develop a taste for something later or sooner. But what you don't want to have happen is for you to sink a bunch of time and money into something in which he doesn't really want to do or participate in, but is afraid to say no to his grandpa (for a multitude of reasons, as I am sure you can imagine).
That said, one would think that if he truly were curious about this kind of stuff, you would have seen (or heard of from his parents) this kind of interest; there isn't any reason today why a kid couldn't find a free way to program a computer (amazing stuff can be done with javascript and HTML5 built into every modern browser), or take apart and learn from electronic junk, or read up books from the library (or online) about electronics and robotics, begging his parents for money for parts or other materials...
Do you see what I am getting at here? Today, it is far easier and inexpensive than -ever- for someone who is truly interested in the topic to both learn about it, and participate - for little to no cost - than it ever has been before. So you need to access the situation, and find out if he is truly interested in the topic, and if so - why he hasn't been able to progress any (in programming, if nothing else)...