While your son is at the later end of the spectrum for walking, right now there is not a need for alarm at all. He's showing the signs that he's getting ready to walk. You needn't do anything out of the ordinary. It sounds like you are giving him ample opportunity to move around, stand, etc. As long as he's in an environment where these things can occur, he will walk in due time.
Our eldest grandson did not walk until he was slightly over 20 months. He did not even offer to stand until that age. Needless to say, we were very worried; however, his doctor said his muscle development as well as other physical and mental developments were fine. He does have a "cautious" personality but is not of "inhibited" temperament. By the way, he walked one day and ran the next.
He is now in his fifth year in school and this "cautious" personality has netted him many good friends and a good scholastic record (both academically and behaviorily). Remember, it was the turtle that won the race and not the hare. I wish you the best ..
Is he on target in other areas? Has he begun to talk?
Some healthy normal children have very cautious personalties and are slow to walk out of that sense of caution.
If he's reached all his other milestones on time or early, he sounds just like a variation on the mean. If he's delayed in other areas, you will want to seek early childhood help to have him evaluated and get early therapy if needed - the earlier the better.
Best wishes.