Use a bottle with a small teat opening - you're in Germany right? I always used the ones from DM's own brand (babylove??) and started with the ones for tea - Lisi had to work VERY hard with that and sooner or later got mad, so went on to the ones for milk which have a slightly larger opening, but not large by any means.
I think the secret is to choose one sort and then stick with it trusting that sooner or later your LO will get the knack, rather than changing all the time trying to find one that works perfectly straight away. With the occasional bottle it is extremely unlikely your ds is going to start to prefer it to you, particularly at his age, it's not that he's a newborn anymore.