It depends on what he is interested in. Food, drinks or just visiting places. Singapore has become quite expensive. So ask him to be prepared for a rip off.
Universal Studios
Malls like VivoCity, nex. Quite a lot of malls there.
Marina Bay sands
Esplanade
Songs of the Sea. Laser and water show in sentosa. Kids might like it.
There are some more attractions in sentosa. Ash him to get a map and choose what he likes.
Bird park and Zoo.
Clarke Quay for pubs (Quite expensive)
Reverse bungee jumping in Clarke Quay.
Pagoda street in China town for souvenir shopping
Mustafa in Little India
Funan Mall for Tech related stuff. But its quite expensive.