my son is arriving in Vienna by plane on Sunday 31 March at 20:30. He needs to get to Keszthely, Hungary from there....
Well, it is a bit problematic. Obviously there is no public transport to Keszthely at such time of the day, and it will be even difficult to get to Hungary after 20:30, since last bus to Budapest leaves Vienna airport at 19:55, while last train towards Budapest leaves even earlier and from the city.
As result, we see a few options here:
a) simply spend a night in hostel/hotel in Vienna and continue to Keszthely in the morning
b) catch last train from Vienna to Sopron (Hungarian town near the border), it departs at 21:47 from Vienna main station (Wien hbf.) or 21:58 from Vienna Meidling station, then spend night in Sopron and continue to Keszthely in the morning by direct bus (earliest bus on April 1 leaves Sopron at 5:45am, arr. to Keszthely 8:50)
c) opt for a private/shared shuttle service right after arrival to Vienna airport. It will be rather expensive (100+ euro) and not sure you can even find a company, which will be willing to take your son that late, but it is possible to try, there are a few private companies offering shuttle service from Vienna to Keszthely
We would probably recommend option A, since it is not very expensive (prices for night in hostel can be as low as 15 euro) and more or less convenient. However options B & C are also on the table, so it's totally your choice. If you decide to go with option A, then let us know, we will try to describe in details how to get form Vienna to Keszthely with the morning start