8st 2lb isn't heavy kawii, don't worry about it. I think you are more worried about sitting on his lap in case you are being a bit 'forward' as you haven't been going out with him long. Give it time and you will know when the time is right to jump on his lap - you are definitely not too heavy ! Good luck.
It doesn't matter how much you weigh, it's whether he wants to do it. from your earlier post, he's not ready for this sort of thing yet. You'll know when, don't rush it.