Jun is just overthinking things, but as you said it's due to him being in a new relationship and still young thus he doesn't have confidence in himself. But I do believe when he got Satoshi and Sho's support, he'll grow in confidence especially if he got into the modelling job, which is a good opportunity to spend his time and to mingle with others.
The smut is great! Even with only Jun and Sho the hotness level is off the chart XD So Satoshi, please come home soon