What kind of doctor sees somebody for osteopenia?

Many. Always start with your primary care physician. Other physicians who treat osteopenia are: endocrinologists, rheumatologists, orthopedics, geriatricians, obgyn...
PCP or rheumatol. Usually the pcp ( primary care physician) may discuss with you about calcium 1 to 1.5 grams a day and vitamin d 800 ui ( if levels are normal >30) also low dosis alendronate 35 mg per week id not contraindicated or no biphosphonates at all. Exercises are also good ( lift weight).