You still need to use さん. Failing to do so would usually make you look like a two-faced person. In general, you should not change the way you refer to someone by name based on whether he/she is present. Well, actually, people sometimes drop さん intentionally when they speak ill of someone, for example, but let's not do this while you are a beginner :)
As an ...
Your observation is correct. In real life, people almost always use ○○先生 or ○○さん rather than ○○博士. Calling someone ○○博士 or 博士 is something we see in fiction (e.g., オーキド博士 in Pokémon) and educational TV programs. ○○教授 is sometimes used in real academic fields.