Ok OP, I'll be back in a moment. Sorry for not getting back to you, I have been busy homeschooling and working from home!
Cardinal numbers are basically just looking at a number and knowing it's number...and it's good for developing a sense of number I.e. 5+1=6..... instead it counting 1,2,3,4,5,6 by fingers. I can hold 5 in my head and onwards to 1.
Numicon is good for developing a sense of cardinals.... I.e. the 5 shape looks like a 5... he would then be able to look at the shape and say whether it's a 6 7 or not. I will buy a numicon set from Amazon.
When working on maths, it's good to start from within 10, then build up to 20... e.g. 2+4, 6+3 etc. You really do not want him to count on his fingers or do the whole counting by route... you want him to hold a number in his head and then count onwards. Numicon will help with this. When adding to numbers together... e.g.2+3..: you don't count the numicon shapes for example I.e. 1,2,3,4,5,6. You will say, ok I've got 2 in my head and I will count onwards "3,4,5".
I don't know if this is making sense. I'm sure it isn't, I have a huge range of apps I can recommend but I'll have to do this next week as I'm swamped! In the meantime, get numicon and start counting within numbers from 10.
To your other query, he could have MLD but this is not always the case.