I have a dynamic Sentinel internet connect going to a RF550vpn and it will not establish a VPN connection.

He is using dhcp and always getting the same ip address. He also said that the dhcp lease was only 7 mins.

VPN will drop and reconnect automatically after DHCP is done. The reason being: DHCP will get new IP address and the VPN connections needs to reestablish. If the DHCP address is always the same IP address, perhaps he can set it as static.
Sprint should not be asking for renew every 7 minutes, this is definitely a problem......