वह मेरे कमरे जा रहा है । - He is going to my room. Explain why मेरे कमरे is used instead of मेर

वह मेरे कमरे जा रहा है । - He is going to my room. Explain why मेरे कमरे is used instead of मेरा कमरा ?

It should be "vah mere kamare mai ja raha hai" 
I believe it's because of 'mai' you change it to "kamare" but if you want to say this is my room then you can use 'mera kamara' like this "Yah mera kamara hai"

You are very welcome John.