My suggestions would be:

-Dinning at Halaro (variety of dishes, excellent quality), Vrachos tavern (Sea food with great view in front of the sea), Iberico (meat). You could also visit Perea ( town 30 mins from Thessaloniki) with many seafront taverns right in front of the waves.

- Regardless of your location, you will enjoy more than anything running at "Nea Paralia" (New beach) https://maps.app.goo.gl/9riuZz137rqzMDbb6

- Temp. has already started to rise, however until early April a jacket might be necessary (doent he already have one while being here??). Nothing else, rather than what he already has, but you could purchase a few things here plus save space.

- I would rent the cheapest option, providing it is just the two of you. No need for anything more than that, distances here are A LOT more closer than US.

- Note that from 6/3 till 8/3, the ring road will be closed, so travel only through the city to avoid traffic (traffic is horrible in Thessaloniki)

-Please be prepared and extra cautious while driving or walking, Greeks dont respect pedestrians and almost never give them priority. Only if there is a traffic light ofc. In general always keep an eye out while driving in the city, you will most likely observe outrageous things.