I would still say that a thank-you gift would be appropriate.
The traditional gift is just like a shower-hostess gift--flowers, sent to the host/hostess to arrive the day before. So that he receives the thank-you in the middle of all the last-minute logistics to make him feel warm and fuzzy about you.
Or whatever equivalent you think would be nice for your old boss, as a person.
Not necessarily of great monetary value, so much as a tad indulgent.
For the business side, a follow-up letter would be nice.