Our son did not use whole sentences till the age of two. He said only the last syllable of particular word. We noticed that he is lagging behind in speaking as soon as he was one year old. I took him to speaking games and the therapist there advised me to speak with his pediatrician. At first, the pediatrician didn't take me seriously but when he was two she referred him to a speech therapist. The speech therapist's first question was: "Did you check his hearing?" Yes, we did. First time we did it, he was just one month old and they observed his eye movement when different sounds started. His hearing was afterwards checked every year.
The speech therapist also ruled out any potential disabilities and conditions, like autism. If she had any doubt he would be referred to a child development specialist.
At the age of three he had a regular appointment with a psychologist (all children at the age of three have it). At that time he was still lagging behind and we continued with speech therapies.
I have to say that nothing that speech therapist said or did we didn't already do ourselves. We spoke to him a lot, read to him a lot, played with him (games like memory, actually everything that we could buy and was age appropriate).
When he was four he already caught up with others and had a vocabulary of a six-year-old or so the test results said.
To sum it up: First consult his pediatrician, check his hearing, go to a speech therapist and see if any further referral is necessary.
Any irregularities in speech development should be taken very seriously.