Speech disorder and delay

[Sparkle41]

DS almost 4.5 has recently been diagnosed with ASD and also has delayed and disordered language. He has speech of a 2.5 year old, lots of nouns, linking 2 sometimes 3 words and some longer phrases, when motivated and wants something (food/TV/I pad) puts longer sentence/phrases together and much better eye contact but otherwise is not conversational and very echolaic (delayed scripting from TV progs/sings/books). He loves the iPad and is above average for phonics/numbers for his age but has difficulty following verbal instruction, much of which I think is due to a lack of motivation. We are doing private SALT as had little help from NHS but I am not seeing much progress. I have been working on letters/phonics/concepts on the I pad and he loves these apps. I try to play and get joint attention with him and get his older NT siblings to do so as well as much as possible. I feel like we are "stuck" at a very basic functional level and unsure of how else I can help him. I know despite wishing for it that there is not going to be a big breakthrough like when a typically developing child learns to speak but wondering if anyone has been there and has any words of wisdom. I have no idea if things will improve, gather pace or not. He is very stubborn and I have no idea if this is personality/result of ASD or frustration at not being able/ not wanting to speak which is not making for happy school holidays! We use lots of visuals and pictures but he is not that interested. Also wondering if the scripting echolalia is typically something that lessons as there is more language or is it a stim? He doesn't have any other stims/sensory issues or obsessions (other than I pad!) Sorry for rambling I just want to do everything possible to help him or even just feel that it will get better and I will one day be able to have a conversation that isn't just a request for something.