Pawpaws are easily grafted in the spring. We graft pawpaws after bud break, when new green growth is just commencing. In Connecticut, this is around May 10th. We have found whip and tongue grafts to be most successful, though chip budding also works.
2-3 year old pawpaw seedlings are the ideal diameter for whip and tongue grafting. One step which seems to key to success, regardless of whether grafts are made onto large trees in the field or smaller potted rootstock in a greenhouse, is to completely wrap the scion in parafilm. This prevents desiccation.
New grafts on young seedling rootstock are vulnerable to winter kill in USDA zone 6 and below and should be protected over their first winter, either in a hoop house or by mounding soil up around the graft union if planted in the field. Grafts made onto seedling trees should begin to produce fruit in 3-5 years.