suddenly a very bad mood and constant howling :D

My 2-year-old boy is suddenly in a bad mood and constantly howling yesterday and today...I have no idea what changed... he almost never howls... he sniffed normally on walks... so he is not searching for a female scent...our theory is that he missed his best friend who has been on vacation for the last few weeks... but can't really tell :\ :\

Health-wise... he seems fine, he eats normally... just super grumpy and howly

Any ideas? and Thanks for help