The only context I can think of where we use in with morning is the single phrase in the morning, which can mean "tomorrow morning", or "the morning of the next day", or a habitual "on a typical morning", or it can qualify a further expression of time ("three weeks from now, in the morning").
In all other contexts we either use no preposition ("I saw him yesterday morning") or use "on".
The iWeb corpus confirms this: in the morning gets 320,615 hits, while all other in * morning together get around 11 thousand hits, but by inspection almost all of these are different constructions: either with in as an adverb or particle on the end of the previous phrase (eg "they came in this morning"), or with morning used attributely with a following noun phrase (eg "we see in our morning papers that ...").
The same applies to evening and afternoon: we can say in the evening, in the afternoon, but in any other context, if there is a preposition it is on.