All very familiar, very similar pattern of development in DS2 (4.4) - found "hard" ones like "how many?" easy and "easy" ones like "what did you do today?" impossible. This is the year when DS2 is cracking it though. I agree with all the advice you've had.
Please tell your DH to take heart. You have lots of good advice on this thread and most of use who are getting through the tunnel feel that well-used visuals are key here. As well as "More than Words", the book I cannot recommend highly enough to you and your DH is "visual strategies for improving communication" by Linday Hodgson. What's particularly valuable in this work is the introductory chapter which explains why visuals work when other things don't. Once you've really grasped what visuals are doing that language isn't, and how you can create "visual bridges", you can tailor-make solutions for your own child and you can explain it all better to staff at school (who tend to think of visual aids as something to "get over" or "get beyond" and who therefore tend to use them inappropriately or not at all). Ideally, you want the TA to read this book too.
If he's making remarks about the present all the time, he may not be far off from chatting about the past once he has the visual backup. For chatting/communicating about past times, I suggest taking photos of all events and buy several of those photo holders that allow you to display 20 or 30 photos on your wall. This is a great visual aid because it doesn't look "therapyish" so outsiders and visitors can join in and comment on them too. Comment on the photos often using phrases that can become a model for your DS eg:
"We bought grapes at the supermarket"
"Remember, we went to the beach?"
"Remember, it snowed at our house!"
"Remember, nana came to our house?"
"Remember, there was a Christmas tree in the sitting room"?
I found "Remember" a really useful word combined with photos of the family's recent history. The trigger-word "remember" told DS2 that we were talking about the past. Then, when he wanted to talk about the past, he could invoke it clearly by just using the one word "remember" ("Remember, nana's trampoline!") even if his grammar wasn't up to doing it verbally. (he still has no idea what "remember" actually means ie opposite of "forget" but that doesn't matter). "Remember, we went to the beach" looking at a photo is a sort of past tense equivalent of "mummy's sitting down" so hopefully he's not too far off that? And if you could get to that, then DH could make up photo albums to mull over together....
Also involving your DH, if you have managed to do family meals, then you can use these to model for DS ie
you:"DH, did you go to work today?"
DH: "Yes, I went to work/no, I didn't go to work today, I stayed at home."
dh: "what did you do today?"
you: I went to work/I picked up DS from school. Did you go to school today DH?
dh: "no, grownups don't go to school. Boys go to school"
etc, etc, ad nauseam over about 6 months but it will all be so worth it in the end.......