Hi Mmitch44,
A mattress that is too firm or too soft for you (or that has developed soft spots or sags in the mattress under the heavier parts of your boy) can cause discomfort and back pain if your spine is not in good alignment in all your sleeping positions but the most common cause of back pain is a mattress that is too soft so your old bed may be too soft.
The first place I would start is the tutorial post here.
Outside of the link there to a list of some of the better online memory foam sources I’m aware of … if you let me know your city or zip code I’d be happy to let you know of any of the better options or possibilities I’m aware of in your area.
Phoenix