I don't see any complication here.
He lends you money and you repay it whenever you can/want to.
If you repay him the just the €15k, then it is an interest free loan and there are no tax implications.
If you pay him interest, then he should declare the interest on his tax return.
If you decide to sell him a share in the business, then it is much more complicated. You will effectively become partners. He will be subject to income tax on any profits made, whether they are paid to him or not.
I think a sum of €15,000 is too small to give him a share in the business. The accounting and legal and tax work around it would be very expensive.
Whatever you finally agree, make sure that you sign an agreement about what you are doing. Sometimes people have a discussion and don't fully agree with what they are actually agreeing . Some other times, years later they forget what they agreed and there is a big dispute over it.
Brendan