إِنَّهُ كَانَ تَوَّابًا
Indeed, He is ever accepting of repentance. (110:3)
Inna adds assertion, -hu is its subject, and kāna tawwāban forms its predicate. Tawwāban is the accusative predicate of kāna; its intensive form conveys abundant acceptance of repentance. Kāna here is not properly reduced to a quality that existed only in a closed past period.
The final declaration gives a reassuring rationale for the order to seek forgiveness. Tawwāb describes Allah as accepting repentance, not as one who himself repents.