Yes, say that he is the reason for your visit.
Saying that you have a boyfriend in the UK will cause immigration to examine your application in case you are going to go underground and stay with him in the UK. However they will not always assume you will do it - it depends on other factors. You have some good factors in your favour:
- You have a job to return to in the Netherlands, and somewhere to live;
- Living conditions in the UK are not much different from the Netherlands, so you have little economic incentive to flee to the UK.
- Your boyfriend is not permanently in the UK, and is in a job where harbouring a fugitive immigrant will cause problems for him.
On the other hand the negatives are:
- you have never met him in person, which makes your relationship less believable. If possible include lots of details about your relationship to show that it is genuine.
- Your host family is sponsoring the trip. Why are they doing that? It's not a normal thing for an employer to do.
So there is a very reasonable chance that your visit will be approved, even if you tell them about your boyfriend.
On the other hand, if you don't tell them about him, and they find out, then you are likely to be facing a long term ban from the UK, meaning you will not get to visit him until his time in the UK is over.