He sounds very much like my ds1, who has quite a severe speech and language delay. My son, though, also has some obsessive/repetitive behaviours, rather like autism but the consultant paed says he isn't autistic... anyway.
He has been having speech therapy since he was 2 (he will be 4 in May) and to be honest, I don't think that it has done much good as regards his actual speech. It has, however, been very useful for learning how to speak to him, and for improving his concentration.
However, his speech has VASTLY improved over time. He was incredibly difficult to understand for a long time - some of his words for things, although they clearly were his words for things, bore no relation at all to the actual word! It was like us learning his language, rather than him learning ours. We called this language "Lewish" (his name is Lewis lol). In addition, he used to stress the wrong syllables of words, and his diction was strange.
All this has improved over time. He now seems to want to say things correctly - it never seemed to bother him at all! We found that what helped him most is not putting any pressure on him, but after, for instance, he had said "Daddun gonna word" we would say "Yes, Daddy has gone to work" etc. They do take it in, even when they don't seem to be!
The upside of having a child who doesn't speak that well is that if you have an Essex accent, you end up speaking much more "nicely", and if you were fairly well-spoken to start with, you end up sounding like the Queen.....