If my child is a late talker, will sign language make him less likely to use speech?

No.  Children are very smart.  They will do what ever is easiest to get their needs met.

If your child is a late talker, he may have even developed some bad habits like yelling, throwing a tantrum, or whining to get his needs met.  Giving him another, more appropriate way, to communicate will decrease his frustration (and yours).

Once speech and language develop, sign usage will decrease.