Monoplegia Cerebral Palsy in 19 month old and arachnoid cyst?

My son is nearly 19 1/2 months old and he can say mum, dad, bub, nan and yum (5 words), he doesn't seem to be even attempting to really make many sounds. He didn't start sitting up until 7 1/2 months, crawling at 10 1/2 months and just started walking at 18 months old. He went in for an MRI last week because he has low muscle tone in one leg, the leg with low muscle tone(left leg) is also longer than the other leg by nearly 2cm. He limps really badly, walking with the longer leg out to the side, his walks on the inside of his ankle as well. Has anyone heard of anything like this before and if so, please post your thoughts, thanks. ADDED: This was the post I put up last week, but I have been given a diagnosis since then, I have been told my little boy have monoplegia cerebral palsy (Left leg only) and upon looking at MRI have found an arachnoid cyst, should i be worried about this?? Apparently completely unrelated to his leg problem. It is slightly smaller than the size of a poker chip (sorry im in NZ and thought it was pointless comparing to our money, i guess poker chips are the same all around the world??) He also has a large hard lump (feels like bone) just under his knee booked in for an xray soon, any ideas on what this could be and anyone else experienced anything like this? Seems to be a few different things which I am being told all unrelated to each other. Thank you. Drs seem a bit baffled, will keep doing tests until absolutely sure of whats going on in his little body.