As it's his house and you are basically paying to live there as a lodger, albeit one who is in a relationship with the landlord, do you think you'd ever really feel secure in the house and that it is just as much your home as his?
Neither one of you knows how this is going to pan out, which is fine. Neither one of you has to commit to a definite plan for the future - that's what living together is all about discovering. However, you might want to consider moving in on the basis that if all goes swimmingly, you choose a new place to buy together with both your names on the new mortgage after a year or two together in his house. If he's already paid lots of mortgage contributions and has significant equity, he can still protect that, so if he's unwilling to countenance the idea, I'd be very wary of his motives in asking you to move in.
Also, have you had the big chats about marriage/children/money? Talking about them doesn't mean you expect them to happen right away, but there's no point in moving in with someone if you want marriage and DC ultimately and he never sees them in the picture at all. You need to know you have the same goals in life, even if it turns out that you don't want them with each other and disagree on the timescales.
Also be mindful that you will need full financial transparency. Once you share an address, your finances may well become linked, and if either one of you has a damaged credit rating, that could affect the other.
Provided all those things are sorted, I'd say go for it. With no dependents and your own income, this is a risk you can afford to take and could be hugely enjoyable even if it doesn't work out long term. 4 months in one relationship can be equivalent to a year in another - it all depends on how much you see each other and how well you communicate.