Mum-tum's are common, he's almost certainly used to them, he clearly likes you and if he wants to sleep with you, it's not going to be a shock or turn-off to him, he's unlikely to even notice.
And, even though he's physically fit, there's a better than average chance that he's thinking about sex and hoping you won't notice any physical flaws that he feels self-conscious about. They don't like to show it but I can't tell you the amount of male friends and ex partners who were convinced they were hideous because of love handles, back acne, beer belly etc.
Also, another option is if you lie down the tummy blends in and doesn't sag at all and there's lots of fun variations on the missionary that aren't too intimidating or adventurous for the first time.
But if you're self-conscious, try not to take it all too seriously. Keep it lighthearted: chat and gently joke about it to keep it from being awkward and make it fun instead of a big emotional thing.
Sex is inherently undignified, filled with lots of weird squelchy noises, stupid faces and clumsy manoeuvring. The best sex is never tidy, pretty or graceful :). Just enjoy playing around, keep it fun and try not to worry about your belly - I can guarantee you he won't be 