Also I'd like to add a little about love and it's relationship to a patronus. As we know from canon, a patronus is a guardian and is summoned by happy memories/notions. It can also take the form of a loved one or represent a loved one to the caster.
Well, as Jo herself said that Tom never loved anyone in his life and does not undertand it and therefore always underestimates it, this could very well mean that he wasn't able to. What with he thinking himself so superior, he probably told himself that he didn't need one while actually, had he tried, he would have found out that he really can't cast it.
The love and purity needed to cast a patronus probably has to do with the purity of the caster's heart and soul which we know he lacked. A patronus, I also believe is more powerful when used to protect others rather then the caster alone, and he would never do that. We know that Dolores could cast one and maybe it's just me but it appeared a pretty weak on as apposed to Harry's, who cast it to protect all the people in the room.