ert1569

Contributing $$ is not an issue. Transfer bank to bank is easiest. Keep a signed note of what it’s for .. signed by both of you .. and consider the consequences of if you and your partner end up not together in the US. “That won’t happen” I hear you say .. but even the best laid plans can go sideways .. best to be upfront and on the same page and in agreement with what will happen in all the “ifs” situations. A simply written legal agreement that reflects what you agree to do once you marry ( add you to the title ) or if you separate (.. he repays you ??? ) would be how I would proceed..  IMO

@ert1569, with all due respect - I hope that you weigh all your options and carefully consider the advice both solicited and unsolicited that people are giving you here. 
I would personally take one step at a time and focus on the spousal visa first. You are in for a long wait. Everything is taking longer these days with COVID. 

Now check with the title company. 

Co-owning a home with your spouse will make it
 
* easier to get a spousal visa.  
 
* harder to visit under a visa waiver 

Many places the mortgage document holder must match the deed ownership.   You need to verify if your plan is even doable from a legal perspective