He is 1 year and 8 months old. Note recognising and saying is no problem. He knows a few useful words but he really only uses "mummy" and "daddy" for communication. It would be great if he could ask "car" for the type of toy that he wants but he knows to say "car" as generic vehicle descriptor. He uses "mummy" and "daddy" but it is real frustrating for me and him that he can't ask for what he wants. When should I expect him to transition to requests?
I don't know that I think concern is the right word for what you should be, aware might be better. I have 3 kids. One talked early, the other two would be considered severely speech delayed. In neither case did we end up with any special therapy. We just worked on it & when it clicked, it was very rapid.
The fact that we did not opt for therapy in our situation is only to say that therapy is not always necessary or desirable. However, knowing what I knew of speech development, any child that isn't linking words by 18months likely should be looked at just to ensure there isn't any underlying reasons. Ideally by 18 months you are looking to see 2-3 word combinations, simple things. They may say things like "me stay", or "up momma". Those kinds of statements. If they cannot do that, you should speak to the doctor. I did with mine as well. A hearing check is in order as well as a few other minor things they will run through to determine if all other things seem up to par.
So personally I wouldn't be concerned. I wouldn't. I would however be properly aware that this is a little behind & take the steps to ensure there is not something I could do that would assist them in this area.
When my 1st talked very early & extremely clearly, it set me up to be overly concerned when my 2nd talked extremely late. My 1st said at his 1st bday "Happy birthday me!", my second was saying momma & dadda & bubba (for brother). Right up until almost three, he had some words, but mostly useless things that didn't help us communicate at all. He knew the word snake for example. Where we live you would rarely if ever see one. We don't own any. He had no reason to use the word other than liking snakes. So to me, this was a frustration as he still didn't use words like "eat" or "hungry". He could say please, but most of the time I didn't know what he was asking for. Please, on it's own, isn't terribly useful either. After all that worry & concern (& reassurances by his Dr that he was doing great in all other aspects & he believed this would resolve) my late talker went from not talking at all (other than momma, dada, snake, please, etc) at 3yrs old, to talking in full conversations within a few months & a few months later had taught himself to fully read. He could read better out loud than most kids I knew more than twice his age. I was unaware he had even been learning when I heard him read something out loud, so I called him over & handed him a cereal box & asked him to read a line of text & he did so without any pause.
All that you should be concerned about at this age is making sure he has no barriers that could be removed for him (such as a need for ear tubes or the like). If he otherwise seems completely cognitively on par, then speech on it's own is rarely a significant issue & most children will sort it out by 2.5-3yrs. I have watched a lot of kids, come from a large family, have my own kids & to date, every kid I've known that has talked late, still does eventually talk & talks just fine. The major concerns are only when there are other concerning factors linked with also not speaking well for age. In such cases, early intervention is much more important. It is so important in helping a child catch up before they are hitting school age. That is why you also see a Dr. They see so many children and they are trained to know anything that might signify a more serious concern & what is just a deviation on typical. The vast majority of the time there is nothing more going on, so don't be concerned until you have been given a reason to be, but do be alert & never be afraid to get a Dr's opinion on the situation. When my 3rd was also a late talker I was far less worried than with my 2nd, as I'd seen it before, but I still had her checked a few times to ensure there was nothing I was missing. I wasn't worried. I was just doing my due diligence in ensuring I hadn't missed anything.
If he's 1yr 8 months , that's 20 months if you're very concerned you should basically visit your pediatrician, other than that give him time and communicate with him, buy him the first 100 books and starts showing him the pictures as you name them and ask him to repeat.
Based on my experience , my boy is trilingual and he didn't have words until almost 2yrs. I bought puzzles, small story books and blocks anything that can make me talk to him more and engage him. Within a month he had 10 words. He could come to me and pinpoint things and call them by the name.
This really helped. So it's something you can try. I also believe you'll get a speech therapist if you're in the state's... not sure if all states give this service, you might want to find more from your pediatrician.