@Allison is 100% correct about this. You won't have to talk to him much at all. Ostensibly the grown-ups will be paying attention to the birthday baby and visiting with each other. Your DNiece and this BF will be be focused on each other and/or their phones. Believe me - this
is how it will happen.
Unfortunately there isn't much way to nuance it; at least if you don't want to risk them misunderstanding and bringing him anyway. For example, if you say "We were actually thinking it would just be family", it may be too vague and they just shrug and bring him anyway. You'll need to be direct and say "No, not this time."
You don't have to let him come; you really don't. I'm all for setting and protecting boundaries but I do urge you to be realistic. If he's not included be prepared for your niece to get petulant and refuse to come herself. Teens really, really are like this and as a parent, not all hills are worth dying on. If her parents allow her to bow out, don't get bitter.

Whatever happens, try not to stress - enjoy the day with your precious little one.