Hi Gorta - my ds (5.5yo) also has a severe language disorder (his is associated with ASD). In the last 2+ years he has had 4 CELFs done (first the preschool CELF & now the 5+ one) at 6 monthly intervals.
1st test - un-testable, he could not do anything at all
2nd test - receptive 0.1st & 0.2nd centiles, expressive 1st & 2nd centiles
3rd test - receptive 1st & 2nd centiles, expressive 9th & 18th centiles
4th test (3 months ago, this was the first time he has been tested on the 5+ CELF test) - receptive 9th centile, expressive 32nd. (sorry can't remember the subtests)
He has just had a test of comprehension where he tested ahead of his peers (as he does in picture & vocabulary tests) though his language is still disordered & he has problems with some very basic elements (pronouns, some prepositions, sequencing etc).
He made considerable progress between the tests & he was very severe (entirely mute at 3 years old with no receptive language at all).
We did ABA at home for 2 years - nowhere near full time probably averaging about 10 hours (or fewer) a week (& we had to work on lots of different delays not just the language issues though they were our primary concern). We did only naturalistic teaching with masses of opportunities & reinforcement for ds to speak. tbh SALT has never actually done anything - we just use her for assessments. As well as the ABA we've used More Than Words, with lots of sign (Signalong) support & some visuals. We're using Language for Thinking atm & Sentence Builder & Question Builder on the ipad. Starting to learn to read is helping with some of his grammar issues but we have a very long way to go.
I guess I'm saying that there can be masses of good progress, but the problems won't magically get better - I can't imagine ds ever having typical language - and it will take enormous work & you are going to have to do an awful of that yourself.