Switch back to rice for a few days and see if he suddenly does well again. Otherwise, it could be a tooth coming in, growth spurt, a new level of awareness that is overwhelming, or so many other things. But if you switch back to rice and he is better, that would be a pretty good clue that he can't tolerate the multigrain kind [which contains wheat]. Also, it is my understanding that wheat should not be introduced until at least 8 months [some say much longer since it is an allergen], so it might be a good idea to just wait on the multigrain kind for a while regardless of whether or not that is what's causing his behavior.
Here is a great site for finding out what foods to give baby and when, as well as recipes, how to make homemade baby food, nutrition info, etc:
http://www.wholesomebabyfood.com/
You can also ask your doctor.
There are certain foods that really should not be given to babies- you probably know the obvious honey, chocolate, nuts, etc. but there are others you might not have thought about [like strawberries, which are also highly allergenic]. It's important to know because if you start a food too soon, the baby could actually develop an allergy to it, whereas if you had introduced the same food later on when the child's body was ready for it, it might not cause any issues. It's exciting to let your baby try new foods and see what their reaction is and to see what they really love and hate- but it's not worth the risk of causing food allergies when you could wait a few months and lessen the chance of any allergies developing.