It is common for male names to be put into the second declension when Latinized, in which case they inflect in general like any other second-declension noun. So going from  *Raonīus* as a second-declension nominative form, it is quite clear that the accusative is *Raonīum* and the ablative/dative is *Raonīō.*

As far as I know, no native Latin name ends in *-īus* with a long vowel, so it might be a little hard to find analogies for the genitive and vocative. (Some names taken from foreign languages, most often Greek, end like this; e.g. *Dārīus.*). I think the usual pattern will most likely apply in this case, so the genitive will be *Raonīī* and the vocative *Raonīe.*

The declension of names ending in *-ius* with a **short** i show the following complications:

- In the genitive singular, *-ī* can show up instead of *-iī*. The position of stress in contracted genitive forms ending in *-ī* is supposed to be the same as in uncontracted forms in *-iī.* It's not clear to me how this kind of contraction would even apply in the case of your friend's name, so I would just go with *-īī.* (The final vowel is long, although resources often don't mark that because it is fairly easy to predict once you know a little bit of Latin.)

- Names ending in *-ius* in the nominative singular typically take *-ī* in the vocative. As in the genitive, the stress in *-ī* vocatives is supposed to be on the same syllable as in the nominative. However, vocative *-ie* also exists. Its distribution is a little tricky to describe, and is discussed in "O Egregie Grammatice: The Vocative Problems of Latin Words Ending in -ius", by Eleanor Dickey (2000). Since your proposal *Raonīus* ends in *-īus* and not *-ius,* I would recommend forming the vocative as *Raonīe.*