Dave, (my 3-4 ft. Eves needle) decided the joy of standing erect, no longer interested him. However, since he layed down, a few pups have shown up, so thats a good sign. What's going on though? Do I need a soil change? He's 3-4 years old and has been standing tall all that time, then last month, just lost all his energy. He's in full sun most of the day and hasn't been moved, watering him only when the soil is dry.

Do i need to amputate or just let Dave be Dave?

I would suggest check the base of the plant and see if the part touching the soil has gotten too soft/rotted. If it is not, then just stake the plant to help it go upright.

This cacti is rather thirstier than most cacti especially when grown outdoors. It appreciates direct sun but if temps are too hot it may not grow leaves and just keep its tall cylindrical trunk. Given more light it may also branch out with arms up and down the trunk or grow leaves. Grown outdoors step up watering during summer, but make sure media is much more grittier and well.draining. The hotter it is the more watering needed at intervals, especially if temps hits 90F to triple digits Less water during colder months.

It won't stand up straight again and I doubt you would be happy with the results of staking as a whole cluster of new branches has grown at the end and would suddenly be growing at right angles to the plant. Has that cluster rooted into the ground? If it hasn't, put a pot under and let it root, then you can sever it from the first plant. If it has, sever it, dig it up and pot it.

Eve's Needle is a fast growing cactus. After 3 - 4 years, I would expect it to completely fill the pot. Maybe its trying to tell you its not getting as much sun as it needs. Growing towards brighter light would certainly be a clue.