1. Dear *****sie, the first thing you should rest assured about is that your fiance will not suddenly be sent back to Portugal. This will not happen. However, your fiance will have to regularise his situation in the UK. This he can do you getting a Permanent Residence card and then one year after that, he can apply for UK citizenship based on being in the UK residing here continuously for more than five years. However, he will need to have a Permanent Residence card for one year before he can apply for citizenship.
2. Here is a link to the application form for a Permanent Residence card https://www.gov.uk/government/publications/apply-for-a-document-certifying-permanent-residence-or-permanent-residence-card-form-eea-pr. He will need this for one year to show he has been living visa free int he UK for one year before applying for UK citizenship. Once he has British citizenship he may live in the uK forever.
3. Here is a link on the UK Government website which sets out how to apply for citizenship https://www.gov.uk/becoming-a-british-citizen/check-if-you-can-apply
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