When will he start forming sentences?

From around 12 months, language development explodes, with the memorisation and sentence construction stages.  From 18 months, some children will be using 20 new words a day and yours will start making associations between himself and other things, making himself understood even if he doesn’t quite have all the words he needs in his proto-sentences.  Over the next few months, his sentences will get longer and more complex, you can encourage him by gently helping him to repeat difficult words.  Don’t pretend if you don’t understand what he’s saying!  At the age of two, your child will already know between 100 and 250 words, but children who are young for their age and those born prematurely are often late talkers.