Had a laminectomy in 1988 at l5/s1. in 1991 x-ray revealed curvature at 3/4 and 5. loss of disc height at l5/s1. is this why i can't put on weight?

No: Degenerative spine disease should have no effect on someone's weight. Such changes in the spine are very common and generally don't require any special care unless there is neurologic impairment. Check your BMI online. If you are underweight despite a good diet, talk to your doctor. It's not the spine.

Interesting question: Superficial answer: no. However, details might influence the answer. Most typically, patients with back problems tend to gain weight (due to inactivity and stress-related eating). So, your biology is different. What is your inherent weight biology (historical weight character)? A smaller percent of people tend to loose weight with stress. Out of space.