I personally wouldn't worry. At 15 months he's still quite little on the language development front. My DD at that age probably said about that number of words, and now at 18 months it's more like 30. It happens really quickly. It took her ages to say Mummy - apparently it's not that uncommon!
In my group there are babies who use a great deal fewer words than DD at 18 months, and those whose parents report they already use three-word sentences (I am a little bit
about this, but they definitely think they do!) It's just really variable, but I was told by the HV that by and large they all get to exactly the same point by about 3 years old.
This is apparently what you might expect by 18 months. Your DS has got ages to get to this point though and already has at least eight words which seems good going!
Children develop skills at different rates, but by 18 months, usually children will:
Enjoy games like peek-a-boo and pat-a-cake and toys that make a noise.
Start to understand a few simple words, like ‘drink’, ‘shoe’ and ‘car’. Also simple instructions like 'kiss mummy', 'kick ball' and 'give me'.
Point to things when asked, like familiar people and objects such as ‘book’ and ‘car’.
Use up to 20 simple words, such as 'cup', 'daddy' and 'dog'. These words may not always be easily recognised by unfamiliar adults.
Gesture or point, often with words or sounds to show what they want.
Copy lots of things that adults say and gestures that they make.
Start to enjoy simple pretend play, for example pretending to talk on the phone.