Your son has a great vocabulary for his age!! Don't worry, he will be picking up more words and saying them more clearly over the next few months. My oldest son said "not really" when he was 15 months old, and I asked him if he was ready to get out of his high chair. I thought I was hearing things!! :) He was talking in complete sentences by 2 1/2. He is 16 now.
My "little guy", on the other hand, is a completely different story. He is 2 1/2, and doesn't say but maybe 10 words total, and never all 10 on the same day...much less making any sentences!
The "jibberish" you are hearing is actually the beginning phase of his speaking. Keep working with your son, and as you see things, point them out to him and say "bird", "flower", "sky", "sun", "light", etc. Then have him repeat the word you just said. Don't forget to include colors and numbers, too. By this time next year, you may even be wondering "how can I shut him up???" (We ALL go thru that as moms!) Be careful, though...since this is the beginning of the age where they do repeat what they hear, you certainly don't want to be using "bad" words in his presence! I guarantee he'll be saying them fast!
Welcome to the FUN stage of being a MOM!!! :) :) :)