What Causes Abdominal Pain In A Child Post Appendectomy?

My 4 year old son had an appendectomy 7 days ago, every time he eats he gets stomach pains about 20 minutes after and he still has the pain in the centre of his groin whilst urinating. (This is the pain I took him to the doctors for in the first place, a pain directly above his penis) I ve also noticed his penis is erect a lot of the time since having his operation. I m just wondering if all this is normal... (If it helps the surgeon said 1/4 of his appendix was gangrenous, he was on 5 days of IV antibiotics 3 different ones, and had an open surgery rather than keyhole)