Keep in mind that a well-sealed house will have minimal humidification requirements compared to a leaky one. There is no significant difference between the source of heat, forced air, radiant in floor, radiant baseboard ,etc in the humidity except that these might affect air leakage (forced air with ducts outside of the house envelope). I expect that you will find that in the same geographic area, compared to an old, leaky home, you will find that on those real cold days is the only time you will want the humidifier. I don't think you can use the most common humidifiers in AC ducts when they are not used for heating. You are likely to get condensation running out of the ducts. I could be wrong, there might be a way to do it.

After the 2016 - 2017 first overwintering of a couple citrus in my basement and the eventual presence of spider mites by the end of winter, I decided to run a small evaporative humidifier in my basement during the winter of 2017 - 2018. I had it sortof centered in the middle of the cluster of trees. I also had a large oscillating pedestal fan running down there too, generally 24/7, but I would cut it off for a day or two each week and would also change its position so that it could hit different spots of the growing area with a varying intensity (it was generally kept about 6 - 8 ft from the trees and other plants down there, so they weren't blasted). That seemed to have kept the spider mites away all winter (but then I had an aphid issue then). Since I have full central air (heat and AC), I closed off the vents in the basement near the plants for this season in order to reduce the hot air issue, but this past year (2018) had been so excessively rainy here that I have yet to turn the humidifier or the fan on down there during the entire 2018 - 2019 winter (so far). I have humidifiers on the 1st floor that I have currently running (including a "whole house" one positioned near a central air intake) but have also not run one on the 2nd floor (my bedroom has been fine but the other bedroom with the trees has tended to be drier so I have been experimenting with daily misting). So I am sortof in an anecdotal limbo which illustrates all the variables involved and how changing conditions might impact how you control the room environment. Would agree with Mike about a hygrometer (I have temp/humidity probes all over the house and outside) although I know Nancy has one too. I aim to have the various areas in the 40% - 50% range, where the 40s are fine for me. As a note, in winter during cold/dry snaps without the humidifiers, my house has actually gotten down to as low as 11% for humidity, so I have had to be vigilant in keeping the humidity up to at least the 30%s, if anything for the furniture and to guard against static electricity. Alternately in the summer, a dehumidifier is definitely needed although fortunately my central air unit actually dehumidifies (there is a tube going from it to the stationary tub in the basement where the water drains) and it generally keeps the house in the 40% - 50% range (with the air off during "cooler" but humid or rainy days, it can get as high as the 60% - 80% range).

Low humidity in the winter is usually caused by leakage issues. It is disappointing to hear your new construction is suffering from low humidity. Even a tight house can become dry when the outdoor temperature and humidity are low. How large is your house, and what size is the furnace? Does the furnace cycle on and off for short period of times? If it does, then the furnace is not running long enough for the humidifier to raise the humidity. I think it is possible to wire your humidifier to turn on your furnace blower whenever the humidity is low. This would avoid spending another $2800 on a second humidifier. If you have an Aprilaire humidifier then it may be possible to install a Aprilaire 60 humidstat with blower activation. I have no experience with this set up, but it is worth investigating if you have a compatible humidstat and furnace.

"He told me get a humidifier installed to mitigate the problem as it will persist even after he fixes it." It can't hurt as that is a bit low. But what is he proposing to fix it? What type of wood? Finish? Subfloor? Custom or spec house?