My question is can we use the source code of the software that we made for previous customer and we use that source code again legally without his permission? can he take any legal action ?
That would depend on the contract between you and your previous customer. If the contract says the customer has the copyright then no. If the contract says you have the copyright then yes. If the contract says nothing then ask your lawyer.
Can he take any legal action? In the USA, yes, absolutely. The customer can take legal action if he doesn't like your hair style. Whether they have a chance to win is another matter. But if your contract said nothing about copyright, then they can take legal action, which costs you time and money even if you win, and in court you never know what happens.