21 month old speech

[Sayers]

My son understands most things said to him and can follow directions like give this to your dad, throw your diaper away or touch your nose, toes etc.  He even can eat with a fork or spoon.  However, his speech seems delayed as he only says babble type words.  He can say bye, shoe, cat, mom, and dad.  It is hard to get him to communicate clearly.  Is this normal?  Should I take him to a speech therapist? He has also fallen a number of times and incurred bruises to his forhead could that have caused any permanent development delays.  Additionally, he suffered from RSV and has a bit of ashthma would that contribute to speech delays.  He has trouble pronouncing words and when his sister says "B" he replies back saying "P"

[JRREsq]

My son was diagnosed at 19 months with a significant speech delay as he fell below the 1st percentile for expressive language at his age.  He was only really babbling, but kind of said Mum, DaAnd would never mimic sounds He had great receptive language and cognitive development all tested within normal limits.  
I had him tested by our county Early Intervention program.  As far as I know every state/county has this program for 0-3 year olds.  You can call and tell them your concerns and they will set up an evaluation.  Depending on the results they will determine if you qualify for services. [not finance based, just child need based qualification]
My son obviously qualified and has had speech therapy since he was 19 months.
He is now 30 months and has about 100 words as well as some 2 word sentences.  He still very very difficult to understand and will likely need to continue his therapy after he "graduates" out of the Early Intervention at 3.  But there are programs after that as well.
Also, a great book to read that really helped me to help him was "Teach Me How to Say It Right: Helping Your Child With Articulation Problems" by Dorothy P. Dougherty.  My son's speech therapist let me read her copy and I liked it so much I bought my own.  
Good luck!