I think if the repayment includes interest (as in your example), that interest will be considered your taxable income and you'll have to pay tax on it (and universal charge probably).
If the loan is interest-free, the foregone interest will be considered your gift to your son and will be potentially subject to gift tax. However, at these relatively small amounts the tax will not become immediately payable, and is going to be reduced or eliminated by the small annual exemption, though not sure of the exact figure of the exemption, might be around 3,000 euro per year that can be gifted without counting towards future gift/inheritance tax.
I think that's the situation but might be wrong, so hope someone more knowledgeable will answer your question better.