My 2 mo old has weighed 13.0 lbs for 3 weeks and has only had one bm in 2 wks. Concern? He's breastfed

Yes. A 2 month-old baby who has not gained weight for 2-3 weeks needs to be seen by a doctor right away. Babies that age gain about 1 ounce a day, so a baby that is not gaining is probably ill with some medical condition, or is not getting fed enough. A primary care doctor's evaluation can figure out the cause. If such a baby seems weak or limp, parents should call 911 for an ambulance.
Yes. Healthy breastfed babies may have infrequent bowel movements even up to two weeks, although unusual. But they are usually active, wetting their diapers frequently and gaining weight. The combination of poor weight gain in addition to infrequent stools makes me think that there is inadequate caloric and fluid intake. Formula supplementation my be necessary.