Since writing an obituary for a citizen of Verona before the invention of the printing press is certainly not going to be in the form that contemporary obituaries are written, the writer can take some license in form and detail. First of all, Romeo's age has not been indicated as has Juliet's, who "hath not seen the change of fourteen years." The only indication of Romeo's age is from Romeus and Juliet, a work to which Shakespeare referred for the writing of his play:
"One Romeus, who was of race a Montague,
Upon whose tender chin, as yet, no manlike beard there grew,
Whose beauty and whose shape so far the rest did stain"
Many critics, thus, place Romeo's age as somewhere between fifteen and eighteen. In addition to having no beard, he is so impetuous and ingenuous that he cannot be older. With the setting as the fourteenth century, the writer can place Romeo as having been born in 1345, perhaps, and having died in 1361. And since the day is very torrid in Act III, Scene 1, perhaps Romeo has died in summer.
Keeping in mind that Romeo has died during the Italian Renaissance, the obituary may be rather flowery and expansive in thought. For instance, some dramatization of the events leading to Romeo's death may be mentioned. And, certainly, the untimeliness of his and Juliet's death must be noted and bemoaned. The fact that Italians have a penchant for incorporating passion into their works will allow for emotive wording. Referring to the Prologue, the writer of Romeo's obituary can incorporate the tropes of this verse, as well. Also, the words of Juliet in the balcony scene, represent Romeo's life and may be a good opener:
It is too rash, too unadvis'd, too sudden;
Too like the lightning, which doth cease to be
Ere one can say ‘It lightens.’ Sweet, good night! (2.2.124-126)
Perhaps, then, the writer of the obituary can compose an obituary that is a little more than just the facts of Romeo's death. Referring to the local newspaper and its Obituary page will be of assistance, too.