Hmm… tough question. I started math before I started reading because he was 29 months the first time I read the book and he said the “cut off” age was 30 months… so I was like “yikes!” and started right away. We didn’t have any problems introducing reading though, he loved seeing all of his family members’ names written down and enjoyed reading.
I think the reason he recommends this is because reading is the base of all future learning, and hence the most important, so it would be on the top of your priority list. But I don’t think it will make that much of a difference. HOWEVER, when it comes to the encyclopedic knowledge program, I would probably introduce reading and math before that because after looking at paintings by Van Gogh, rock specimens, vehicle models and butterfly species, looking at a bunch of red dots, or a bunch of red lines (words) won’t be all that interesting and it may be a little more difficult to get him interested (it’s kind of like how they say to introduce babies to vegetables before introducing fruits, because if you do it the other way around they will often not take to the vegetables because they’re used to the sweetness of the fruit).
And also, about going back to the cards, there really won’t be a need to if you go through the whole math program. By the time you’ve done six months of equations (or more, or less), he will have seen each number many, many times, and will have a firm grasp of what numbers are and how they relate to each other. And really I think the only names they really have to remember is like 1-19, and then 20, 30, 40, 50, 60, 70, 80, 90, 100 (since, like “97” is 90 + 7, we don’t really have all that much original names in English, so he’s really not memorizing 100 different names, more like 30) If you continue teaching him math - if you go through equations, problem-solving, teaching numerals, and beyond to continue learning with him in his journey with math - if it’s a part of his every day life, he will never forget the numbers.
And about starting both at the same time, you might want to try a more gradual approach, as to not overwhelm him OR you. It doesn’t have to be a drastic difference - considering his age I’d maybe have at least a week’s space between the two. It’s not like it has to be six months or anything, you just don’t want to go from no program to a full-blow program in one day.