If you did not choose the Black Eagles class to start, it may be more difficult to make the optimal Ferdinand build you want. However, you can still recruit him if Byleth has an armor rank of C and a high Dex stat. Having meals with him or doing Choir Practice and inviting Ferdinand to have tea can also be efficient ways to raise support points with him. Higher support ranks with Ferdinand make it much easier to recruit him and do not necessitate getting Byleth’s ranks and stats so high. Recruiting Ferdinand without support ranks is especially hard, because training Byleth’s heavy armor rank in the early game is tricky, compared to other skills that have more characters who can train Byleth.