He was not able to conjure one. His soul was split and you need positive memories for a patronus. When he made the horcruxes he destroyed himself, this makes it impossible for him to conjure a patronus. By the way, he was like irresistible against Dementors, what should he need a patronus for? They followed his orders, they weren't about to attack him or his death eaters. All of them except Snape weren't able too, they had no need for it.

what about before making horcruxes....he must have petronus when he was in hogwarts

Snake?

Before splitting his soul or becoming the dark wizard that he was, he was a young normal boy. He must have had some form of patronus and I too think it would be a snake...

He could have or might have not had one. As what Dumbledore said in Deathly Hallows, "That which Voldemort does not value, he takes no trouble to comprehend." Perhaps he could have been capable of conjuring up a patronus, but he never really cared to learn how or even understand what a patronus is coz to him, it is of no worth or significance.

He was not interested in defending magic. He was a murderer, an attacker a dark wizard. He had no need, not at Hogwarts and not later. Never.

He wasn't able to preform one.

He didn't need one, with the dementors as his Allies.

Most people don't have patronuses. They are very complicated magic. I don't think Voldemort would have had any really happy memories to draw on, even before he split his soul.