Hi Nobother
First of all, well done on taking a practical approach to it. Most people enter into such transactions without making any contingency plans. Then if they do split up, the whole thing becomes a financial nightmare which makes the whole thing much worse.
He wants to be able to contribute to the mortgage, but understandably wants to be protected for this contribution should things ever sour
Option 1
If he is your tenant paying rent only, then he needs no protection. As Jambo points out, if he pays you rent up to the Rent a Room maximum, you won't pay tax on it, and he will get some small tax relief.
I would probably suggest this as the initial step. It requires no solicitor or major financial transaction. You probably should do a simple agreement "This represents rent only. It does not convey any ownership on the property or any liability for the mortgage". He should pay this into your bank account and not into the mortage account.
Option 2 Boyfriend buys half your apartment at current market value.
Property value today| €300k
Mortgage| €400k
BF's share of house|50%
BF's share of mortgage|37.5%
At its simplest, if your BF has €150k available, he could buy half your apartment from you and you would set that off against the mortgage reducing it to €250k.
He would end up with half an apartment worth €150k
You would end up with half an apartment worth €150k with a mortgage of €250k.
If he does not have €150k available, he could be added to the mortgage and title deeds, and you would do a legal agreement to reflect the above.
This is technically possible but I would advise your boyfriend not to do it. If things don't work out, then he would find it very difficult to unwind. It is not a good idea to go into partnership with someone with a €100k deficit.
There would be legal and stamp duty costs, but what would it achieve? Nothing as far as I can see.
Option 1A
If you are struggling with your mortgage, your boyfriend can pay you rent and can lend you money. You pay him that back when you can. If you split up , he might have difficulty getting it back.
Conclusion
The simplest is for you to just charge him rent for the moment. If you split up there are no financial hassles.
The most likely medium term outcome is that you sell both your houses and buy a larger house together. If you can't sell your apartment because of negative equity, your boyfriend may buy the house in his own name. We will deal with those complications when it arises.