Understanding and speech are entirely different elements of communication. It is not too early to have a speech delay at 15 months.
Words themselves aren't a concern at this age, but the crucial thing is whether your DS is babbling well? Lots of vowel sounds initially, then some consonants. If he isn't using recognisable words, but is babbling, with a nice varied tone and clear sounds, then I would agree with previous posters that you shouldn't be too concerned.
However, if he is not babbling at all, or only uses one or two sounds, then get a referral to SALT.
DD1 only had two sounds at 8 months. It was monitored, but ignored because she had improved mildly by 10 months. Now, at 4.9, she is delayed by over a year, and attends a special school. She has a brain malformation, missed until she was 2.9 because nothing was taken seriously until preschool waved a massive red flag when she joined.
I am NOT saying your child has SN
but if he has any delay, it is worth getting it checked now, because a little input at an early age is worth months of input later.
Also, you could do makaton signing. My DD3 is 17 months old, and has lots of signs because she sees me doing it with DD1, and watches Something Special. It supports speech development really well, and encourages it.