Loose stool / constipation / not eating regularly

I have a 11 1/2 year old Bichon Frise that has recently start having problems with loose stool / constipation and not eating regularly in the past week or so.  We have no idea what the problem is.  Out of the past week he has had a couple of days where he has energy and wants to play, but for the most part, he hasn't been acting himself.  

Does anyone know what might be causing this?

[Wooly25]

Try feeding him 3 parts cooked rice to 1 part cotage cheese.  He problemly isn't acting right because he is uncomfortable.  He also may be having teeth problems some older dogs have problems eating food and chewying it enough if they have bad teeth.