Connecting through PPPShar Pro proxy

My son has been playing MUDs through MushClient via a dial-up connection successfully. Recently we have established a household network where he connects to internet, email, etc. through my computer which has the dial-up access. We haven't been able to establish a connection - we just get the "Connection refused or failed" error message.

Here are the details:

Son is running MushClient under WinXP.
His network IP address: 192.168.1.1 / 255.255.255.0
Connects to my computer via 10/100 Ethernet hub.
I am running Win98SE.
My network IP address: 192.168.1.5 /255/255/255/0
I connect to internet via 56k dial-up modem.
I run PPPShar Pro proxy.
All normal protocols operate on their standard ports through the proxy, eg. HTTP, HTTPS, FTP, Socks, SMTP.
PPPShar Pro proxy settings include Telnet on port 23.
Son has no trouble using www, MSN Messenger, etc.
He has added the proxy server IP address (192.168.1.5) to his MushClient configuration and tried it with both Socks4 and Socks5 specified.

We are both running ZoneAlarm firewalls but so far as I can see I have allowed all the accesses he needs to get through. I think I remember once we tried connecting with the firewalls disabled but without success.

Any more ideas?

Since www works I would guess that the proxy needs to be configured to allow the MUD port through. Haven't heard of it so can't be more helpful, but would look at the proxy configuration.