at 12 weeks old, it is probably a growth spurt, if he is bf, you need to feed him more & if he is bottle, up the amount in the day time bottles, babies this young it is normal for them to go differing lengths at night at times, if he is sleeping well in the day (naps of 1.5-2 hours not 30-45mins) then he has good sleep habits & he needs feeding.
HTH