Specific language impairment is usually specific to that child. For instance, you say he has receptive problems - where do these problems lie? Does he essentially have an auditory processing disorder - ie his hearing is fine, but the brain cannot accurately interpret the sounds coming in? A child might for instance hear - cat, mat, pat, hat all as "a". They don't process the consonants around it. They have to use context to guess that actually we mean cat here. Obviously, if the representation in the brain for cat is just "a", then they are going to have problems reading and spelling the word. Likewise, if they never hear "s", "ed", "ing" at the end of a word, they are never going to work out their function in grammar (hence a syntactical disorder) and have problems with reading and spelling.
Has the speech therapist said if he has a phonological processing disorder?
One of the other problems with a receptive disorder, which make its impact more long term than just an expressive disorder, is that the ability to form concepts is impaired - this will obviously cause more problems as they move up through the education system, when language gets more complex and abstract.
So, really the nature of the specific language impairment and its comorbid condtions can show up as dyslexia - which is likewise an umbrella term for a whole range of problems with reading, spelling and writing. Children with an auditory processing type disorder, who struggle with the building blocks of sounds, words and language may have dyslexia, but while reading is a struggle, they may get the gist of the story. Children with a top down processing disorder in language may have what sound like good words, good decoding but then struggle to get the meaning or gist of the story.
For instance, he may have poor auditory sequential memory - if you told him to go upstairs, get his socks, come down and put them on? This will impact on spelling (phonological problems apart) and writing, when he may struggle to keep the order of the words in the sentence in his working memory long enough to spell and write them down. Writing has a heavy cognitive load, because of the need to process the question, formulate an answer, assemble it grammatically, keep it in working memory long enough to write it down in the right order; spell the words and concentrate on the fine motor skills to write it legibly.
Tambaboy talks for instance about her son's struggle with blending.
He might though have very good visual memory, and learn to read words using advanced shape recognition (which most of us do, once we have moved past c-a-t gives cat. We don't have to break down and blend every word we read, as adults).
If English is his first language, I'd look at getting an English speech therapist to assess his language; and then get an educational psychologist, specialising in dyslexia to assess his other learning skills - to find out exactly what is going on in his language; and the knock on effects in reading, spelling and writing; and possibly maths. Some children can do arithmetic, but once you start wrapping up in language such as "Janet has 2 kgs of flour, but needs 3 kgs to make a loaf; how much does she need to buy?", they get lost in the language.
I can't give you a positive story, but if you explain his problems to him, so he realises its not because he is stupid; and give him strategies to get round the problems, it will help his self esteem - and help considerably through the education system. Also, tell him what he is good at and dyslexia is nothing to be ashamed of. I found "How to Detect and Manage Dyslexia" by Philomena Ott easy to read, with some good explanations and lots of strategies to get round the problems.