His decision should be based on several factors:
Budget: Can he afford the cost of a new tux, or is it more economical to rent one and buy one when he can afford it down the road?
Other Events: If you and your groom will be invited to a number of events in the near future (weddings of friends/relatives, business and charity functions) than it may be better to buy a new one. Renting a tux for every occasion will add up and the cost of buying now may be more affordable in the long run.
Suit Style: For looks that aren’t the classic black tux but have tails or cutaway jackets you may want to rent since you’re not likely to wear this specific style again.
You should also visit both retailers and rental shops and try on different styles to see what looks best on him as well as what all the costs are. Factor in prices for the style of pant and jacket, all the accessories (buttons, cufflinks, ties, vest, cummerbund) and for the number of days you’re going to be renting it.