HI there, could you post his routine? He may be ready for an A increase. 45 mins can be developmental, or can be UT for some kids. They are tired enough to get to sleep but really can't do more than 1 sleep cycle because they feel too rested. If he's started fighting naps I would guess it is UT at his age. However, if he's sick then you really just have to follow his cues and ride it out until he's feeling better.
Hugs!