I would also suggest you talk to your friend before gifting his son the bike,
it is possible tat your friend doesn't want his son to ride a bike to start off with,
in which case it would be wrong for you to go ahead and get his son the bike, and Allah forbid if the boy is involved in an accident and get's injured or worse, you'll have to live with the guilt and your friend won't be too pleased with you either.
I would recommend you to first talk to your friend and see what his views are on his son owning and riding a bike as this is an extremely dangerous undertaking in the traffic of Karachi, bikers drive like crazy and accidents are commonplace, in these accidents, serious injuries and fatalities have a very high probability due to lack of protective clothing and helmets etc,
does the boy own a bike currently or have access to a bike, does he know how to ride and does he have much experience riding,
if the response to any of the above is negative, then any form of a motorbike would potentially be dangerous and I would first seek the fathers consent, and then give the boy a 70cc with the promise of an upgrade in a couple of years provided he treats it responsibly and doesn't have any mishaps due to his own negligence or bravado.
As for a 250cc bike, these can be seriously powerful, it's like a kid asking for a bb gun and handing him an AK47 which can potentially be a recipie for disaster.
Irrespective of your decision, i would urge you to seek consent from the boys father first prior to doing anything else.