When will his head fill in??

[Mojave's Mom]

Mojave is 18 months old and is still at a stage where his head doesn't look in proportion with his body. He still has a really cute puppy look to him but I'm wondering around what age his head will "fill in" to look in proportion to the rest of his body? About 6 months ago or so he was at a stage where everything looked in proportion but then he filled out body-wise and his head looks smaller....are they full-grown at around 2 years or 3 years of age?

Thanks for any information in advance!!