My LO was similar, and it was a concern for the nurses too when we went for our 18mo routine checkup at polyclinic - we ended up being referred to child development unit for a development assessment, where we found out my son had isolated speech delay.
He's 2 years old tomorrow and he only knows "mama" and "mummmum" at the moment, but he's been improving a lot! While I don't know about your child, you can highlight it to your PD/polyclinic when you go for the next checkup and get a referral to child development unit to do an assessment :)
Anonymous