I think you can rule out birde for now cos serious bird shots usually photog go alone and spend hours hiding in the bushes ... unless you wanna hide in the bushes with him.
Depending on how prepared you are / how much you want to do, you could draw up a simple plan something like:
1) 7am - meet and go to one of the reservoirs - lots of scenery to shoot. Have a quiet, simple breakfast at the resevoir itself. Can also let him practise his portrait and poses shooting with you as a model. LOL! Sometime lucky, also got wild animals. For the breakfast, pack a simple picnic like sandwiches lovingly made by you (or his fave BF food), and have pet bottles of frozen drink (non-aerated) to make it through the humid morning. Be sure to pick up after you makan.
2) Move off by 10-11 am as it would be too hot already to hang around reservoir, then start moving to Little India or China Town. Little India is preferred cos still got a lot of authenticity where as China Town already totally refurbished and commercialised.
Do the famous Nasi Brayni lunch and teh tarik at Tekka Market Hawker Center (not the eateries along Lil India), and at the same time, shoot the hawker center candids. Move off and be sure to explore all the little and back lanes in Lil India. Sunday being a market day, it would be a riot of colours as most of the stalls would be fully stocked. Again, tons of photo op for streets, candids, buildings. Hungry or thirsty all along the way got shiok food and drinks. This should provide enough photo op for the entire day, but if you want variety ... by the time you go from the Tekka Market start to almost the end near Mustafa Center, you can take the Farrer Park NEL ...
3) Move off again by 4.30pm latest and take NEL/MRT go to the Eastern End of the island. Take bus to Changi Point/Village in time for sunset photo op. Along the way, there would be interesting scenery and if lucky, some birds (?)
4) By the time sunset over, move to the near-by Changi Village where there are many small cafes and have a cosy dinner. If you're not in the mood for slow and romantic, but want gung ho, noisy and shiok, go the the hawker center.
After dinner can stick around and explore the surroundings, or go to one of the drinking holes if he likes. I don't think Charlie's at the back of the hawker center is still there, but the other outdoor 'pub' might still be, so the both of you could finish the evening with cold drinks under the stars.
For #4, actually if you do dinner at one of the cafes, you could plan a small surprise party by calling his close kakis or photo kakis and organise a simple bash. Or even if you have dinner at the hawker center, why not be really different and give him a birthday to remember? Celebrate it with tables of his buddies around, do a tame version of a table or lap dance for him and then do the birthday cake thing in the hawker center itself! Make sure there's someone else taking pix of the celebrations.
By the time everything's over, guarantee he pengz liao.
I'm sure other CSers will have better suggestions, but basically you want to provide him with the most photo ops within each location, and the difference parts of the island for variety.