My fianc was working and heard something pop in lower back since then he has had very bad pain and spasms hard to move. what could this be and what?

Back issues: A "pop" in the back can be from strain/sprain, herniation or even fracture. A full history and physical needs to be performed and possibly imaging. Likely would benefit from anti-inflammatory, pain reliever, and even muscle relaxer. Osteopathic physicians can offer additional skill of musculoskeletal manipulation if appropriate to relieve discomfort. Must be seen for eval.

Disc issue or muscle: Likely muscle strain but possibly an aggravated disc. Very common in younger people. Conservative measures like heat and nsaids. Then consider therapy and traction. A steroid injection may help as well. Over 90% of people get better on their own but aggressive early treatment works best. Avoid prolonged bed rest.