They are almost interchangeable, but por is more common since it's typically used for a more general time (think "sometime in the morning") whereas "en la mañana" is used more for a very specific (but unspecified) time. Also there is "de la mañana" which is typically used for specific points in time.
Here are some examples that I hope clarifies a bit:
- «Fui a la tienda en la mañana del 28 de agosto» -- "I went to the store on the morning of August 28th" (An unspecified, but specific point in time is being referred to)
- «Voy a ir a la tienda por la mañana» -- "I'm going to the store in the morning" (A general point in time -- think sometime in the morning)
- «Son las trés de la mañana» -- "It's 3 in the morning" (A specific point in time)
My Spanish is quite well developed, and I'm particularly adept at translating into English. As far as I'm concerned, the most coherent translation is: he introduces/will introduce himself tomorrow morning.
However, as someone pointed out, it's a broken sentence so trying to extract any more technical meaning is a waste of time.
To whom does he present what tomorrow morning? Would DL accept "He is making a presentation..."? Presentar is shown in the dictionaries as also meaning "propose" or "submit". Friends have been arguing about when to meet and "he proposes tomorrow morning". We have to make some sense or we can't translate.
The word present can either be used as a noun or as a verb.
When used as a noun:
- It is synonymous with "gift" -- "We all love to open presents on our birthday!"
When used as a verb, it can mean one of the following three things:
- To give something to someone in a formal way -- "I present this award to you"
- To formally talk about something to a group of people (To make a presentation in your words) -- "He presents tomorrow morning"
- To make something available to be used or considered -- "He presented a check for $10,000 to his favorite charity"