congrats on the new lungfish!
i would agree with the comments posted above, and you can take your own variations on a lot of the suggestions as you see what works for your specific situation, lungfish, and tank setup.
i have kept the P.annectens lungfish for several years (was the first species i picked up) and have really enjoyed this species.
in the case of your setup, at least until you can get a larger tank, i would go with moderate feedings and frequent water changes. a lungfish of this size does not need to eat very often, and you will be fine to feed it just 2-3 times a week, with several pieces of tilapia fillet (if you want to keep with what he was being fed before), but may also want to mix it up with some shrimp pellets, earthworm sticks (kensfish.com makes a good earthworm pellet that i have found all lungfish species to enjoy), carnivore pellets and some vegetable matter (algae wafers and veggie sticks are a good supplement to their meaty foods, and seem to go a long way towards healing up/preventing lateral line erosion, which African lungfishes are prone to).
with a feeding schedule of about 2-3 times per week, i would suggest a 50% water change every other week, or more frequently if you can do so. lungfishes will do very well with frequent large water changes. filtration should be high, and be prepared to net out large waste matter from the lungfish if the filter doesnt seem to be picking it up.
i've kept all of the African lungfishes, and found the annectens and the aethiopicus to be most aggressive (dolloi seems to be the most shy, and the amphibius seems to be the most sociable...still...) i highly suggest keeping any african lungfish species in its own tank as you never know when they will strike at a tankmate. if you are looking for a lungfish species that gets along with tankmates, you may want to consider the South American lungfish or the Australian lungfish (both of which can be kept with a variety of tankmates).
floating plants do well with lungfish setups, and planted species seem to be readily uprooted (and sometimes eaten); i've kept lungfishes on a variety of substrates, but currently have kept my Africans on a heavy sand and they do very well on it. sounds like your gravel will work out well though, so stick with what works!
good luck, and let us know if you have any further questions--
--solomon