Can we vaccinate a 2-year-old boy with functional or anatomic asplenia against meningococcal disease if he has not completed a series of PCV13?

Yes, depending on the brand you use. If you are using Menveo (MenACWY-CRM, GSK) or MenQuadfi (MenACWY-TT, Sanofi Pasteur), they may be administered at the same time as PCV13 or at any interval before or after receipt of PCV13. If you are going to give him Menactra (MenACWY-D, Sanofi Pasteur), you need to wait at least 4 weeks after he completes the PCV13 series before giving him the Menactra to avoid the possibility of interference with the immune response to PCV13.