It is very common for babies around this age to go on nursing strikes. Moms often mistake it for signs of weaning, but it is only temporary. It is a busy age and sometimes babies simply get distracted. It could also be from teething (sore gums) or during or after illness (sore throat, stuffy nose, ear infection) - things that make nursing uncomfortable. Some babies will refuse to nurse if they had a scare during a recent feed - slammed door, dog bark, mom yelled at dog, etc.
There are some things you can try to bring him back to the breast. Try to nurse him when he's sleepy (just woke up or getting ready for sleep) in a quiet room away from distractions. Encourage a lot of skin to skin contact with him (take baths together, nap together). Make every feeding a positive experience - never try to force one.
Here is an article on nursing strikes:
http://www.llli.org/FAQ/strike.html
You can contact La Leche League or a lactation consultant, too. Good luck! :)