What is the morphological difference between Leydig cell in human and pig?
Perhaps what your teacher meant was not so much a difference in Leydig cell morphology, but in interstitial tissue morphology, ie. tissue which occupies the space in between seminiferous tubules. Leydig cells are its most interesting component, others being small blood vessels (a lot of them), nerves and connective tissue (mostly fibroblasts, mastocytes, lymphocytes and macrophages). In different species interstitial tissue looks differently, for example:


Leydig cells form clumps around blood vessels, and are surrounded by fibroblasts (which makes them more difficult to isolate) in guinea pig, but not in mice or rat; 
in human a lot of interstitial tissue is extracellular matter, Leydig cells are not so frequent, but more active;
and, last but not least, in pig, there is little extracelluar matter and a lot of Leydig cells that are very easily isolated (and produce large amounts of estrogen). 


I found all this in some old lecture notes; I couldn't dig up any sensible reference, though.