He will usually have a mortgage broker working for him and a mortgage can be secured in a couple of days. His first step is to have the offer accepted by yourselves. Secondly he needs to get a valuation of the property and thirdly the valuer reports to the mortgage company usually within a couple of days. If all is OK and the valuation comes back OK the lender will usually offer him the mortage - again within a few days. There is no need why you shouldnt be asking his respresentative (solicitor or estate agents) why no-one has been intouch to value the property within 7 days of the offer being accepted.
Dealing with an investment buyer usually works better as they know the business well and the chain usually ends or starts with them - they are not relient on anyone else - but the process is the same.