There's no need for the husband to gift the shares to his wife before it ultimately is gifted to the son. Even if the shares are solely in his name, he can gift shares directly to his son and elect to gift splitting the gift with his wife. It's as if husband gifted shares to the wife first without having to actually go through that step.
If the total value of the gift is less than $30K, meaning with gift splitting that each gives an amount that is equal or less than the annual exclusion of $15K for 2018, then only one gift tax return is required with the split being reported all on the one return.
If the combined total value of the split gifts exceeds the combined total of their annual exclusions, again the $30K, then each will file a gift tax return that reports the gift splitting, and it will also report how much of their share of the unified credit will be used to offset the excess of each of their gifts that exceeds the annual exclusion. Per the from 709 instructions, both returns should be mailed in the same envelope so that they are processed at the same time.