There are a lot of private colleges which run night classes in Singapore. These will fit your work schedule better.
If you look at the newspaper when you arrive, you will find many of these courses being advertised. They are usually degree or diploma programs granted from foreign unis, UK. Aus, NZ, US, Canada, so you get a decent certificate at the end. Just be aware that these are colleges which are local partners of the foreign unis are for-profit businesses, so the cost of the subjects may not be cheap. But you will get flexibility.
Search for names like MDIS, BMC, TMC, Stanfield...and probably a dozen others to see what is available.