With a rod (εν ραβδω). The so-called instrumental use of εν like the Hebrew (1. Samuel 17:43). The shepherd leaned on his rod, staff, walking stick. The paedagogue had his rod also.

Shall I come? (ελθω;). Deliberative subjunctive. Paul gives them the choice. They can have him as their spiritual father or as their paedagogue with a rod.