His fast is not invalidated, provided he spits it out and does not swallow it. This act is mildly disliked because it exposes the fast to the risk of being invalidated, except for a valid reason: such as if the spouse is ill-tempered and she tasted, or he feared being cheated in buying food and had no choice but to buy it, or if she couldn't find anyone to chew food for her child from a menstruating or postpartum woman, as mentioned in Al-Hadiya Al-Alaiya p.163.