First thing to note is that there are three parables given.
Why are they given? The occasion for Jesus to give them is back in chapter 15:1-3. The phrarisees and scribes are grumbling because sinners come to Jesus.
He then tells three stories (keep in mind these are parables - the details are not important but the main point Jesus makes).
Note these similarities -
1) Lost sheep --something is lost (sheep), something is found, the finder rejoices, others rejoice with the finder.
2) Lost coin -- something is lost (coin), it is found, the finder rejoices, everyone rejoices with the finder.
3) Prodigal -- something is lost (son), he is found (comes to the father), everyone rejoices (they have a party). But then we get to a different element than the first two stories, the older brother grumbles because the father receives the sinful younger brother. Does that sound like anyone else? It is exactly like the attitude of the pharisees and scribes in verses 1-3 (the reason Jesus tells these parables).
The meaning of these three parables is plain. The proper attitude should be joy when someone comes to Christ.
It is easy to get bogged down by trying to figure the meaning of each of the things in the story of the prodigal. I don't believe Christ ever intended us to try to place a meaning on every element of this story. Just like today when a preacher gives an illustration, every detail of it is not important - just the main point. I fully believe in the inerrancy of scripture - but I still think that these three parables have a simple meaning.