Kudos to your doctor for being so proactive on your son's behalf! I know plenty of moms who went to their pediatricians repeatedly because their sons were showing classic signs of autism (speech delay, stimming, social deficits, lack of non-verbal communication) and they were all told that he was just a kid and would eventually grow out of it. They didn't. By the time their sons eventually got the autism diagnosis when they were 3 or 4, they lost a lot of time out of their child's life that could have been used for early intervention. So, I'm approaching your situation from a different perspective of thank God that your doctor is more aware and more proactive than a lot of other pediatricians out there.
So your son has a mild language delay. Speech therapy would benefit him and, at this age, the services would be provided to you in your home free of charge. He may grow out of it on his own and it is entirely up to you to choose whether or not you want to pursue getting services for him. It has been my experience that speech therapist are usually very fun and the therapy itself is usually very play-based. It wouldn't hurt your son at all to receive speech therapy -- in fact, he actually might really like it. But that is for you to decide whether or not it is worth pursuing. I just think that you are very lucky to have a doctor who is looking out for your son's best interests and is letting you know what options are available to help your son if necessary.
Just a different perspective on the whole situation.