I would get him brought to slt attention asap, atleast just to get them to have a look and see if they want to refer him. At your children's centre they might have drop ins with speech and language, or else I would probably go through health visitor.
He may well be fine, but better safe than sorry and get him on a waiting list early as possible if you can, in case things don't improve.
You might get some resistance from some hvs about referring at this age as some like the wait and see approach but if you deep down have any concerns then push for it. If he ends up talking before he is seen or before any appointments come up for him then just cancel the appointments, and theres no harm done.
I had concerns about one of my dses speech & various issues from around the same age as your ds, due to various stuff housemoves, new baby etc I left it a few more months before actually getting him referred which I regret because he still can't talk now a year later, and we are still waiting for him to start speech therapy. It's shit how long they make you wait!
I think its a good sign he can make the different sounds though and copy what you are saying, lalalala and rararara are quite difficult sounds! Is he able to copy a full range of sounds from you? i would say thats really positive, from the very little I know of this
Advice we have got, while waiting for the speech therapy is to help communication with signing. Not sure if your ds will be able to do it fully or not at 17 months being quite little but would be worth a try I think. It seems to be helping a lot for my ds, eases frustration to atleast have some method of communication. Might be able to get on a course through your childrens centre, if not i think its really expensive. Even you could just learn some of the basic ones from mr tumble or online, to see whether it seems to be useful for your ds. I would start with just a few of the basic useful ones like drink, eat..
Other advice we had was about 'special time' which is 10 mins one to one per day focused on some activity or toys with them, where you have to speak very simply and clearly in short sentences or even single words and describing what they are doing is supposed to be good for that. Really it seems like the type of thing most parents do anyway just playing with the child but for me the main points were about one to one so not having other dc interupting that time, and about trying to keep speech very simple and clear with the short sentences.
General advice from another leaflet from slt was just about 'bath them in words' ie keep talking a lot through the whole day, the more language they are constantly exposed to the better, it can only help.
Blowing bubbles, sucking from straws and sucking thick consistancies has also been recommended to me for my ds, as it helps to strengthen the mouth muscles? Apparently the fruit puree pouchy things are ideal for this, the baby food type ones
Also there's a dvd made by speech and language therapists, which was recommended by someone else on here the other week, called Sookie and Finn. I got that and it seems to be quite good, they do the simple short sentences thing, showing choices, repetition.. might be worth a try.
I'm sure there are more ideas but cant think what else now.. sure someone else will add some more suggestions. Hope your ds does manage to get communicating with you a bit more and is able to ease his frustration, it can be quite hard for them when they cant express themselves