First of all he is a caster so getting
RoB, Urn of Shadows or MoC early plus a Bracer could be a really good choice.
Arcane together with one of the items mentioned previously should be your "core"
Now, Trant Portector is really flexible on his item build, in other words get what you feel your team needs (Remember you don't want to play him has a carry!)
You could put that Bracer in to work and get a Janggo
Get a Mekkans if your team needs one (And you feel that you can finish it early)
If your team already have Mekkans you could get Pipe instead
RoA's slowing effect together with Leech Seed can hold an enemy in place for your team to pin him down
Has for utility items, both Forcestaff, Ghost Staff Blademail and Euls are good options
Finally has for lategame luxuries, the first item on my mind would be Bloodstone, you can build from Arcane has Treant benefits from everything that it gives.
Other options are: