This may or may not be normal. I urge you to get him evaluated by an specialist. The school district is mandated by law to provide evaluation and treatment for any condition that will hinder a child in school. Their services are free. Call your local school district office for a phone number to Early Childhood Intervention office or ask your pediatrician for their number.
If you have insurance to cover a visit to a developmental pediatrician, I would also get an appointment there. If your son has a speech delay, it's absolutely important to get him help as soon as possible.
My grandson is now 7 and is still unable to speak clearly. He has used complete sentences for the past year or so which is really late too. He's been diagnosed with apraxia of speech which means he's unable to use his mouth and lips correctly to form sounds. His mother didn't have him evaluated until he was 2 1/2. Turns out that for his particular situation, earlier diagnosis and treatment may have helped.
My grandson has serious emotional issues with which to contend also. We just recently learned about the developmental pediatrician. He's had insurance to cover his evaluation with her but his mother was unaware that he needed additional help. His age is an added difficulty in getting him able to be successful in life as well as in school. He is remaining in Special Ed for second grade although he's intelligent.
You have nothing to lose and everything to gain by having him evaluated. You will be reassured if he's just a late talker. If he has a physical cause for his delayed speech you will be as early as you could be getting him the treatment that he needs.