How can you prevent poor social skills in someone with learning disabilities?

Find a counselor . Who works on teaching social skills, or find a social skills group therapy group.
Find their strenght. It is common for kids to reflect feelings of inadequacy when they encounter weaknesses in learning that they can't control.The school years come at a period when they are forming their self image. Yet school focuses on a narrow skillset.To counter this effect, find & encourage their pursuit of a strength they have and enjoy.No one worries if eric clapton is dyslexic, or rembrant poor at math.
Support. If you make sure that your child gets appropriate supports for the learning disabilities, and has opportunity to participate in extracurricular activitivities with appropriate supervision to make sure social problems do not develop, you are doinging what is necessary. If any psychosocial issues develop, consult a psychologist.