Your son's speech is brilliant for a 15m old.
I often refer to this chart:
http://www.speech-language-therapy.com/Table4.htm for my son. He is 5years and has issues with the /l/ sound: it nearly always comes out as either /y/,/w/ or /r/. I have had two occasions where we have chatted about soemthing for an hour only to find that the "pilots" were actually "pirates", ditto lorry/lolly.
Some sounds are physically beyond their ability to say - their mouths are just not co-ordinated enough. On the chart you will see that the /ch/ is aquired by 75% by 4 years old.
I like his adoration of Tractors! my DS was obsessed with cars and then Thomas the Tank engine from 6/7 months until he was between 2 and 3.
