They say that the best way to learn a language is to sleep with a native speaker of that language
Being a linguist and language teacher myself, I'm not sure how true that statement is, but it would be unscientific not to test out the hypothesis
Re advice, I would say if he has some knowledge of the language and can express himself even in a basic manner then that is more important than being 100% grammatically accurate etc.
Also, like other posters have said, if you try to learn some Portuguese he will appreciate it and so will you as there will be some extra words to bridge the gap.
The hardest part of Portuguese is the pronunciation. The grammar is similar to other latin languages (do you speak any Spanish?) and is not too problematic. Depending how much time you have to dedicate to learning Portuguese, I know from experience that you can get beyond the "beginner" stage after about 80 hours'/1 semester's worth of study.
I'm sorry I can't offer any phrases that you could text to the new beau in your life as I only studied Portuguese for 1 semester and my knowledge has dwindled due to making space for other languages
Regarding helping him with his English. It will help a lot if you "check" what you say and how you say it. While he's still learning, it is best to use easier words and expressions and when talking face to face, make sure you speak a little slower than usual and make an effort with pronunciation. I don't mean talk to him like he's a moron, but rather talk to him normally but with more attention on vocabulary use and pronunciation.
I don't know what else I can offer you on the subject, or indeed if this reply is of any help at all, but hopefully you will find at least something of what I said useful
Take care
