Hi wembles,
I’ve never been a stickler on keeping all the threads exactly in line with one topic so there’s no problem posting here.
Are you looking to match the height of just the box spring of your older son’s bed or the mattress/foundation combination (so the top surface is the same height)?
If you are looking to match just the box spring height then I would measure the thickness and choose a foundation that matches it (there are several “standard” heights for a box spring or foundation but they will generally be about 8" - 10"). This is assuming that the frame that the box spring is sitting on is also the same height.
If you want to match the top surface of the mattresses, then you would need to measure the height from the top of your older son’s mattress to the floor and subtract the height of the Ultimate Dreams and the frame and that would leave you with the height you need for the foundation.
There are some good foundation options in the foundation thread here. Since the Ultimate Dreams is a polyfoam/latex hybrid then it’s not as important that the spacing between the slats are 3" or less so any of them would be fine.
Phoenix