Your boyfriend is an Argentine national (non-visa national) who contemplates spending 18 continuous months in the UK by combining VISITOR and T5 Sportsperson visas.
For your first question: is it possible to switch out of visitor into a T5 Sportsperson visa? Briefly, the answer is no. It is not possible to switch out of visitor into ANY category. Otherwise there would be a conflict in Paragraph 41 (ii) of the rules, which says in part...
intends to leave the United Kingdom at the end of the period of the
visit as stated by him;
Moreover, since a T5 visa requires entry clearance from all nationalities, Paragraph 28 says in part...
An applicant for an entry clearance must be outside the United Kingdom
and Islands at the time of the application.
And in the current era, the government views even an attempt by a non-visa national to switch in-country as abuse. They don't like it and tend to get upset.
For your second question: does a visitor have to show a return flight? Technically, the answer is no, it is not required. However, we have to return to Paragraph 41 of the rules, sub paragraph (vii), which says...
can meet the cost of the return or onward journey;
So the Immigration Officer is entitled to a high level of comfort that the person's financial standing is sufficient to make arrangements for departure. In practice, a person who can demonstrate that they are a High Net Worth Individual (HNWI) or otherwise independently wealthy can meet this requirement. Others will need to show something tangible.
Overall, the strategy of combining the VISITOR category with another category to obtain more time in the UK is seen as abuse of the system, and they would be within reasonable parameters to refuse a T5 application if the applicant had just spent time as a visitor.
Finally, it's always a good idea to discuss these things with the T5 sponsor beforehand. Doing so will avoid the possible embarrassment of jeopardising their Sponsorship License by violating the code of practice. Sponsorship Licenses are hard to get and companies cherish them. Get your sponsor's agreement beforehand.