Arti,
Your conundrum is the product of your competence. Obviously, you are good at what you do - else you wouldn't be invited to join your boss in his new whirlwind adventure.
Have you seen Jerry Maguire? Its the Tom Cruise and Renee Zelweger movie. Exactly same situation as yours! If you haven't, I strongly urge you to watch it. At least the first 45 minutes - minus the romance bits that is!
Arti, whether you do or you don't join your boss in his new venture actually depends on a number of factors - some more important than others.
Some important factors that you must consider include:
1. Your age and your financial security
2. Your financial commitments (mortgage EMIs, credit card debt levels etc)
3. Your family and social commitments (kids, partners, parents, siblings etc)
But, above all - you need to take a long and hard look at yourself - by which I mean your career aspirations and professional goals.
And, along with the above, you also need to be brutally honest with yourself with regards to your risk appetite - your personal and professional risk appetite.
You don't indicate the size and nature of your current company. If its a large enough company that gives you a decent sense of job security with an equally decent career enhancement opportunity, and if you don't have a high risk appetite, my suggestion is stay put where you are.
Conversely, if your risk appetite is high coupled with your sense of adventure with minimal or no financial, social and other commitments, then it is something worth considering.
I also run a small business. And, its very tempting to poach competent people from medium to large organisations - for obvious reasons. But, whilst it may seem great for a Personal Assistant to carry a new title of 'Executive Assistant to the Managing Partner', the dynamics of and opportunities in medium to large businesses are totally different to SME or SMB sectors - particularly for StartUps - which is what your boss' business is.
Look, I can go on and on. I am sure you get the gist here.
I would hate to see a fledgling career destroyed with a youthfully impulsive decision with scant regard to its probably consequences (or not).
I hope this helps you. Good Luck!