You can either opt for re registration of 60% gift deed or
Have a cancellation deed registered between the parties
But if you do cancellation deed then 60% will revert to your father and he will again have to execute and register gift deed for 60% with younger son, thereby again incurring expenses for registration
Younger son can execute a power of attorney in favour of father to comply with registration formalities on his behalf
The cancellation deed can be signed by son in foreign country itself before a public notary or Indian Consulate
The signed document along with power of attorney can then be couriered to your father in India and he can proceed for registration
The power of attorney also needs to be signed before notary/consulate
If gift deed for 60% is again to be registered, then might as well also include a power in PoA to execute and register gift deed on behalf of donee by donor
Another alternative would be a deed of rectification to gift deed to rectify error of another person deposing before registrar instead of the donee son.