Think you need to post a whole body picture of him stood on a table so his shape can be seen, also what size is he (measure to the top of the shoulder.

Even then it may be hard to tell.

The most likely cross would be with breeds that are common or another crossbreed. As the others said due to the longer hair a Yorkie might well be the case if he is JRT size or smaller, but if larger then there are other candidates. If he is undershot (bottom jaw jutting out) then the other parent could b e one of the long coated smaller breeds like Shi Tzu, Lhasa Apso etc.