@Falalalalaaaalalalalaaaa this sounds very much like my DS2, he just could not get rhymes at all no matter how I tried to explain it. I was very concerned about this and convinced it was a sign of dyslexia. However he has now started school aged 7 - we live abroad - and about two weeks before the start of the school term, he was suddenly identifying rhymes correctly out of nowhere. He is starting to pick up reading and is really getting there much quicker than I had anticipated, not fluent yet but getting there. (I'm very pleased about this!)
DH wonders if COVID came at a really crucial time for his development because the strictest lockdowns were when he was about 1.5-2.5 - these are times where children are often observing so much from others and their vocabulary is often exploding, but thinking back to that time, he wasn't really using words much at all, he mainly used long strongs of unintelligible babbling but with the intonation of speech, and when we look back at videos he's mimicking our intonation perfectly and sometimes trying to copy the exact sounds we made as well. I think it can't purely be the lockdowns, because otherwise every child his age would be struggling, but I do wonder whether lockdown happened to coincide with some specific struggle he has.
In our case he is diagnosed with ADHD, but I think that only really got picked up because there is a family history so I knew what to look for and because he was having significant behavioural/social difficulties in nursery, he went through three in the end, not his fault. They assessed him for autism as well but said no, although TBH I do wonder if this will come back as a question later.
I had got into a bit of a rabbithole about Gestalt language processing and wonder if he possibly has something like that because it fits some of his early language development, but his language wasn't technically late, he was within normal developmental guidelines for each milestone although on the later end. Apparently there is some preliminary research going on now into the concept of "dual processors" (ie children who develop language partially in the normal way, and partially in the GLP way).
The way that he processes language is still slightly odd - he mixes up similar-sounding words e.g. today he was talking about a game and kept saying "dunk" when he meant "duck" and was then insistent DH was the one who was wrong when he tried to correct him. And then the other day he suddenly recognised out of nowhere that the word "pour" a liquid and "poor" (poor me/rich and poor) sound the same but have two different meanings, which he's never registered before. It's not an accent thing, because in our accent they sound the same. And then another day he said something like "How come in Minecraft, it's a pickact, but in this game it's a pickaxe?" and then wouldn't accept that it is also a pickaxe in Minecraft.
Something that was flagged up when he was ~6 and still not able to rhyme even though he'd had 9 months of speech therapy and wasn't mixing up sh and s sounds any more, was whether he possibly has an auditory processing disorder. Where we are, they won't assess for this unless they have struggles learning to read and it doesn't gain him any support anyway so it is unlikely we will be able to have this tested but a lot of the things I read about APD fit. He also has difficulty with things like sequencing and really struggles hugely with verbal stories. He can't follow stories without pictures (I wonder if this will change when his reading gets more fluent) and struggles even with comics, his verbal working memory appears very limited - he has OT and they are trying to work on this at the moment. If he tells a story of something that happened at school for example, it's often muddled up to such a degree that it doesn't actually make sense, and it makes it challenging for him to take on feedback about e.g. behaviour or what to do in a situation.
But he can build Lego like a champion and can identify real life bus models from about 100m distance
he's also good with numbers and calculations, and we've been playing Openguessr and he's really picking up on things like spotting road signs from different countries etc. In some ways his brain is absolutely brilliant so I'm really curious to see how he will get on as he gets older.