Hi Somashekhar,
I do not now much about the Viva brand per se, and a large fraction of my reply might be subjective/opinion.
I am hoping that it might help you in some of the decisions, or aspects of some decisions.
The second hand option is really good - even if it isn't for a trek or Giant or so (would be great if you find a higher end one nevertheless)
I suggest that consider that you are buying a bike for about 4 years of use.(Unless there's a younger sibling or a cousin or someone who can take over the bike from him)
After 13 or so, boys usually have a rapid growth phase and he'll outgrow the bike.
So it might be good to buy a reliable model, though not necessarily a high end or very expensive one - and invest more in a model later on, that he might/can use for longer years.
Material is a good consideration - Aluminium/alloy means lighter bikes, and much less risk of rust.
Where is your son going to ride the bike predominantly? IS it within gated communities? Or on roads? Or are you taking him on trails, hills, off roads, mountain biking, etc?
This can help you draw the constraints and compromises.
I have used only a hybrid, and rode public roads mostly (I use the bike predominantly for commute) - but I found it a huge blessings to have foreign made tyres.
Indian makes are more prone to punctures (especially on Bangalore roads - plenty of traps laid there)
Perhaps MTB tyres might be less prone anyways since they are thicker and have larger treads.
Gears too are more useful once the kid is really familiar with cycling, and on longer and more challenging rides. (probably are important in trails/mountain biking though)
Personally I might compromise on gear/no-gears for price reasons, although I completely understand that 10 year old today might not even consider a cycle without gear worthy in anyway :)
Regards
Juny